port: delete exit_ramp_units, invert the format's own rule, and guard a scale-0 leaf

FOUR THINGS, and the first is what MISSION section 3 calls the measure of
progress.

DELETED `exit_ramp_units` AND `exit_ramp_seconds`. They were authored because the
disc had no time slot on a group's final keyframe, so the ramp into it was the
one unknown duration per screen. Under the corrected record layout that keyframe
does not exist -- a group is an 8-byte header then frames x {u32 time; 36-byte
pose} and every pose is timed. VERIFIED DEAD BEFORE DELETING: setting it to 9999
(166 s) moved the boot's transitions by 0.04 s, which is wall-clock jitter, and
both uses in ScreenView are gated on a condition that no longer fires on any of
the export's 866 keyframes.

INVERTED THE FORMAT'S OWN RULE. `check.rs` enforced "the final keyframe has no
`t`; the disc has no time slot there" and FORMAT.md stated it. Both are now
backwards, and the validator fired 150 times on a re-export. I had not run
`check` between pinning the tag and measuring against the oracle -- the pixel
harness was green while the format validator was failing on every screen with a
multi-keyframe group. A correctness harness does not replace a format one; they
fail at different layers.

GUARDED A SCALE-0 LEAF, which the Decoder hit in its own renderer: its leaf
branch marked the element drawn unconditionally while the blit returned early on
zero scale, so a scale-0 leaf suppressed its parent and blanked the element --
live on all four loading screens. This port did not have the bug only because
authored/rendering.json happens not to list pgloading_loop5. That is an accident
of a gate written for another reason, not a defence, so `_draw_leaf` now reports
whether it drew and `_draw` falls back to the parent.

ISOLATED THE PACING QUESTION rather than leaving it as a suspected regression.
Legacy association: publisher 4.70 agrees, developer 3.92 DIFFERS. Corrected:
publisher 4.26 DIFFERS, developer 3.62 agrees. Both misses are ~0.03 s outside a
composite bound. The association traded which screen is marginally out; it did
not regress the pacing.

Bumped the pin c -> d for the parser and audio changes. Its headline renderer
change does not reach this port: sylpheed-cli builds from the workspace crate.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7FiFFFwbvG2uxdcEh8HyF

Cargo.lock

@@ -4627,7 +4627,7 @@ dependencies = [
  "image",
  "serde",
  "serde_json",
- "sylpheed-formats 0.1.0 (git+https://git.mc02.dev/fabi/Sylpheed.git?tag=formats-pin-2026-08-29c)",
+ "sylpheed-formats 0.1.0 (git+https://git.mc02.dev/fabi/Sylpheed.git?tag=formats-pin-2026-08-29d)",
 ]
 
 [[package]]
@@ -4651,7 +4651,7 @@ dependencies = [
 [[package]]
 name = "sylpheed-formats"
 version = "0.1.0"
-source = "git+https://git.mc02.dev/fabi/Sylpheed.git?tag=formats-pin-2026-08-29c#b28dfedf3548200cc7269b762a422e4c2adeaadd"
+source = "git+https://git.mc02.dev/fabi/Sylpheed.git?tag=formats-pin-2026-08-29d#e2640338e149acc903fac551f400a5319950afb5"
 dependencies = [
  "anyhow",
  "binrw",