export: P0 — GP_TITLE's screens and sprites into the open tree

`sylpheed-export export` reads `dat/GP_TITLE.pak`, enumerates its twelve screen
builds, and writes each as one `sylpheed.screen/2` document with its sprite PNGs
beside it. `sylpheed-export check` validates that tree against docs/FORMAT.md
with no disc in hand — the P0 gate is "validates against FORMAT.md", which is
not something anyone can confirm by reading, so it is a program.

Two readings from FORMAT v1 turned out to be wrong and are corrected here rather
than carried:

* The focus sprite does NOT come from the element's `opt ` link. That was
  measured and refuted upstream, and this export shows why plainly: on the main
  menu `opt ` chains ptloop01 -> ptloop02 -> ptbtn01, two decorations and then a
  button. The highlight pairs by sprite NAME instead (ptbtn01.t32 <->
  ptbtn01f.t32), which is the convention HANDOFF blesses and which resolves all
  five main-menu buttons. The raw link is still exported, renamed `opt_link` so
  nothing downstream mistakes it for navigation.
* There are TWO modulate colours in different byte orders, and they multiply.
  v1's single `#rrggbbaa` could not carry both and silently dropped the alpha
  that every fade ramps. They are now `tint_rgba` and `fade_argb`, with the byte
  order in the key name, because getting it backwards is silent and reads as an
  art bug rather than a parse bug.

The exporter takes exactly one authored input: `authored/screen_names.json`,
because the disc does not name its builds and "build 5 is the main menu" is a
measurement (HANDOFF Q2), not a field. Every name it applies is stamped
`name_source: "authored"` with the evidence in `name_why`, and `check` rejects
an authored name that has no `why` — so the derived tree stays honest about
which of its fields is a decision.

Sprites are per screen, not a flat pool: `main_menu` and `extras` both ship a
`ptbase.t32` and they are different pictures.

Checked, not assumed:

* two exports of the same disc are byte-identical;
* five mutations of a valid main_menu.json — a broken paint_order permutation,
  a dangling focus_sprite, a reversed buttons list, a `#rrggbbaa` colour and an
  invented name_source — are each caught with a specific message.

`t` stays raw. Q1 is answered, but the seconds conversion is measured off the
running game and its own finding flags the frame-rate measurement as the part
worth re-testing; if the game presents at 60 Hz every duration halves. One
constant, at P2, in a file that says it is a decision.

authored/screen_names.json

@@ -0,0 +1,45 @@
+{
+  "format": "sylpheed.screen_names/1",
+  "_": [
+    "Which GP_TITLE build is which screen. AUTHORED: the disc does not name its",
+    "builds, so every name here is a decision. The identifications come from",
+    "HANDOFF Q2 (ui-title-build-map.md), which measured four of them against",
+    "framebuffer captures of the running game; the exporter stamps the name into",
+    "the screen file with name_source: \"authored\" so a reader can tell a",
+    "recovered name from an invented one.",
+    "",
+    "`build` is the index into the pak's list of screen builds -- what",
+    "`sylpheed-cli screen --build N` takes -- and is stable as long as the",
+    "enumeration rule is. The screen file also records the pak entry index,",
+    "which is the stronger locator.",
+    "",
+    "Delete an entry here the day the RE agent decodes a name field."
+  ],
+  "archives": {
+    "dat/GP_TITLE.pak": {
+      "2": {
+        "name": "press_start",
+        "why": "HANDOFF Q2: builds 2/3 are the PRESS (A) BUTTON plate -- a build of its own, composited over the title and faded in a beat later. English of the EN/JP pair. Measured against a live capture."
+      },
+      "3": { "name": "press_start_jp", "why": "HANDOFF Q2: the Japanese twin of build 2. Out of scope for this milestone; named so it is not mistaken for a screen we need." },
+      "4": {
+        "name": "title",
+        "why": "HANDOFF Q2: build 4 is the English title art. Measured against a live capture."
+      },
+      "5": {
+        "name": "main_menu",
+        "why": "HANDOFF Q2: builds 5/8 are the five-button main menu; 5 is English. Measured against a live capture. (An earlier reading called 8 a submenu and was withdrawn -- 8 is the Japanese main menu.)"
+      },
+      "6": {
+        "name": "extras",
+        "why": "HANDOFF Q2: builds 6/9 are the EXTRAS submenu, the only submenu inside this archive. Measured against a fresh EXTRAS capture."
+      },
+      "7": { "name": "title_jp", "why": "HANDOFF Q2: the Japanese twin of build 4." },
+      "8": { "name": "main_menu_jp", "why": "HANDOFF Q2: the Japanese twin of build 5." },
+      "9": { "name": "extras_jp", "why": "HANDOFF Q2: the Japanese twin of build 6." }
+    }
+  },
+  "unnamed": {
+    "dat/GP_TITLE.pak": "Builds 0/1 and 10/11 are a DELTASABER / SYLPHEED A.I. plate that was never seen running -- not in the boot path, not on any title-side screen, not in the attract loop (HANDOFF Q2). They export under their build index rather than a name we would be inventing."
+  }
+}