re: timer_set's second argument is a SECOND countdown, not a warning threshold

mission-phase-timers.md left 180 open as "a limit and a warning
threshold is the obvious reading, but not established".  Reading
sub_822639B8 -- ScriptPhase::Update 0x82263528, same dt as the
stopwatch bank -- settles it the other way:

    if running: [+304] += dt
    if armed:   [+308] -= dt   while [+308] > 0
                else [+312] -= dt, clamped at 0

[+312] is never compared with [+308]; it is decremented, and only in
the A<=0 arm.  Two sequential countdowns.  Disc-wide the second
argument is 180 in all 29 timer_set sites while the first varies
(600 x19, 1200 x8, 900, 1800).

Built-ins 123-127 are vtable slots 90-94 on five scalars at
[phase+304..320].  125 and 126 have ZERO call sites in all 28 scripts:
the script arms, starts and stops this clock but never reads it.

Corrects mission-phase-timers.md, which merged this clock with
stopwatch 0 -- timer_resume starts [+304], set_flag(0) one instruction
later starts the stopwatch the timeline's kind=0 reads.

Docs only; all seven ISL artefacts regenerate byte-identical.

docs/re/structures/isl-mission-timer.md

@@ -0,0 +1,122 @@
+# ✅ The MISSION timer is a second, separate clock — and `timer_set`'s second argument is a second COUNTDOWN
+
+`mission-phase-timers.md` left this open: "`timer_set`'s second argument (`180`)
+— a limit and a warning threshold is the obvious reading, but it is **not**
+established." It is established now, and **the obvious reading is wrong**.
+
+This clock has nothing to do with the 32 stopwatches in
+[isl-timers](isl-timers.md); it is five separate scalar fields on the same
+ScriptPhase, driven by its own updater.
+
+## The five fields
+
+Zeroed together by `sub_8225FEF8` (`0x82260084`–`0x82260098`):
+
+| field | type | meaning |
+|---|---|---|
+| `[phase+304]` | f32 | **elapsed**, counts **up** |
+| `[phase+308]` | f32 | **countdown A**, counts **down** |
+| `[phase+312]` | f32 | **countdown B**, counts down *only after A reaches 0* |
+| `[phase+316]` | int | **running** — gates the elapsed counter |
+| `[phase+320]` | int | **armed** — gates the two countdowns |
+
+## The five built-ins
+
+All are stubs into the ScriptPhase vtable, slots 90–94
+(`0x8226C690`–`0x8226C6F0`):
+
+| built-in | slot | what it does | calls disc-wide |
+|---|---|---|---|
+| 127 `timer_set(a, b)` | 94 | `armed = 1; A = (f32)local[0]; B = (f32)local[1]` | 29 |
+| 123 `timer_resume` | 90 | `running = 1` | 40 |
+| 124 `timer_stop` | 91 | `running = 0; armed = 0` | 170 |
+| 125 `timer_reset` | 92 | `elapsed = A = B = 0.0; armed = 0` | **0** |
+| 126 `timer_elapsed` | 93 | returns `elapsed` as a double in `[phase+176]` | **0** |
+
+`timer_set` reads its two arguments as **doubles** at `0(r4)` and `8(r4)`, `r4`
+being the converted locals array — so `local[0]` and `local[1]`.
+
+## ✅ The update — `sub_822639B8(phase, dt)`
+
+Called from **`ScriptPhase::Update` at `0x82263528`**, with **`fmr f1, f30`** —
+the *same* `dt` the stopwatch bank gets at `0x82263480`. So this clock counts
+the same **seconds** ([isl-timers](isl-timers.md)), and `600 / 900 / 1200 / 1800`
+and `180` are 10 / 15 / 20 / 30 minutes and 3 minutes.
+
+```
+822639C8  if [+316] != 0:  [+304] += dt          ; elapsed counts UP
+822639E4  if [+320] != 0:
+822639F4      if [+308] > 0.0:  [+308] -= dt     ; A counts down ...
+82263A00      else:             [+312] -= dt     ; ... then B does
+82263A10                        if [+312] < 0.0: [+312] = 0.0
+```
+
+### 🔴 That refutes "a limit and a warning threshold"
+
+A threshold would be **compared** against the countdown. `[+312]` is never
+compared with `[+308]` anywhere. It is *decremented*, and the `bc 12, gt`
+at `0x822639FC` jumps **away** to the `A -= dt` arm — so `B -= dt` runs **only**
+in the `A <= 0` branch. The two are **sequential countdowns**: a per-mission
+period, then a fixed second period.
+
+## ✅ Every mission agrees, and the second value never varies
+
+29 `timer_set` sites across the disc:
+
+| first argument | sites |
+|---|---|
+| 600 | 19 |
+| 1200 | 8 |
+| 900 | 1 (Stage 26) |
+| 1800 | 1 (Stage 24) |
+| **second argument** | **180 in all 29** |
+
+A per-mission limit that varies, followed by a fixed 3-minute stage. Note this
+constancy is *consistent* with the sequential reading and does not by itself
+establish it — the disassembly above is what does.
+
+## ✅ What it feeds — and what it does not
+
+Each frame the updater allocates a 32-byte message (type tag `0xAB03E5BA`,
+vtable `0x820A8D68`) and posts it to the bus at `[0x828F35DC]`, carrying:
+
+```
++24 = [phase+304]   elapsed
++28 = [phase+312]   countdown B
++16 = running       ([+316] != 0)
++20 = expired       ([+320] == 1 && [+308] <= 0.0)
+```
+
+`expired` is exactly "countdown A has run out" — the point at which B starts.
+
+🔑 **The script never reads this clock.** `timer_elapsed` and `timer_reset` have
+**zero call sites in all 28 scripts**; the script only *arms* (127), *starts*
+(123) and *stops* (124) it. Its only consumer read here is the posted message.
+
+## 🔴 A conflation in `mission-phase-timers.md`, corrected
+
+That page says timer 0 "is started by `timer_set(1200, 180)` then
+`timer_resume`, with a 1200-second limit". Those are **two different clocks
+armed back to back**:
+
+```
+timer_set(1200, 180)   ->  this object: A = 1200, B = 180, armed
+timer_resume           ->  this object: elapsed starts counting
+set_flag(0)            ->  STOPWATCH 0 starts (isl-timers.md)
+```
+
+The timeline's `kind = 0` reads stopwatch 0, not `[phase+304]`. The two start in
+the same coroutine one instruction apart, which is why they read almost the same
+value and the conflation was invisible. **The 1200-second limit belongs to the
+mission timer; it is not stopwatch 0's limit.**
+
+## 🟡 Not settled
+
+* **Who subscribes to `0xAB03E5BA`.** The literal appears at exactly one site —
+  the producer — but `0xAB03` is the high half of *many* message tags in this
+  image and dispatch is not by literal compare, so a single occurrence is **not**
+  evidence that nothing consumes it. Whether running out of time ends the mission
+  is therefore unread.
+* What B counting to zero does. Nothing observed acts on it.
+* Whether `timer_stop`'s 170 sites are all phase teardown, or some are a real
+  mid-mission stop.