fix(export): take the sylpheed-formats dependency by path, not by tag

Removes the last reason this repository depends on its own history.

The pin was deliberate and carried its own exit condition, written into the
comment above it: "revert to the path dependency the day the tag is an ancestor
of `main`." Measured -- `formats-pin-2026-09-01` (e2630413) IS an ancestor of
`main` now, so the condition is met, and the cost the comment named goes away
with it: while the pin held, `sylpheed-cli` built from the workspace crate and
the exporter from the tag, so `tools/port/verify-screen` compared two eras
instead of detecting drift. They read one decoder again.

It also removes a failure mode nobody priced in. Depending on this repo by tag
is what made the #49 history rewrite break the build: every commit was replaced,
the locked rev vanished, and clean checkouts could not resolve it while the
rewriting machine kept working off its `~/.cargo/git` cache (PR #60). A path
dependency cannot fail that way. `Cargo.lock` now has ZERO references to
Sylpheed.git.

⚠️ This moves the exporter across a 243-file decoder change (8 467 insertions),
so it was gated on the full suite rather than a compile:

  45 suites / 377 passed / 0 failed / 14 ignored, cargo exit 0
  45 binaries launched, 45 reported  (a SIGKILLed suite prints no result line
  and would otherwise vanish from the tally)
  corpus report: PRESENT for disc, res3d and iso

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -5197,7 +5197,7 @@ dependencies = [
  "colored",
  "image",
  "indicatif",
- "sylpheed-formats 0.1.0",
+ "sylpheed-formats",
  "texpresso",
  "tokio",
  "tracing",
@@ -5213,7 +5213,7 @@ dependencies = [
  "image",
  "serde",
  "serde_json",
- "sylpheed-formats 0.1.0 (git+https://git.mc02.dev/fabi/Sylpheed.git?tag=formats-pin-2026-09-01)",
+ "sylpheed-formats",
 ]
 
 [[package]]
@@ -5234,25 +5234,6 @@ dependencies = [
  "xdvdfs",
 ]
 
-[[package]]
-name = "sylpheed-formats"
-version = "0.1.0"
-source = "git+https://git.mc02.dev/fabi/Sylpheed.git?tag=formats-pin-2026-09-01#e26304133732792cb1df5563c21df59471f5bf7d"
-dependencies = [
- "anyhow",
- "binrw",
- "flate2",
- "futures",
- "rayon",
- "serde",
- "serde_json",
- "thiserror 2.0.18",
- "tokio",
- "tracing",
- "ttf-parser 0.24.1",
- "xdvdfs",
-]
-
 [[package]]
 name = "sylpheed-ppc"
 version = "0.1.0"
@@ -5276,7 +5257,7 @@ dependencies = [
  "image",
  "rfd",
  "rodio",
- "sylpheed-formats 0.1.0",
+ "sylpheed-formats",
  "thiserror 2.0.18",
  "tracing",
  "tracing-subscriber",