fix(content): pasted screenshots keep their proportions in posts and comments

[theRealBithive]

What changed

A screenshot pasted into the editor is inserted with no dimensions, so the image extension stores its 500x500 defaults with the node. The read-only renderer turned those into a bare aspect-ratio: 500 / 500, which overrides the image's own proportions once it has loaded, so every wide screenshot was squashed into a square in posts, comments, changelog entries and the widget. The editor never showed it, because it only sets a maximum width.

The renderer now writes aspect-ratio: auto W / H: the stored box is reserved until the image has loaded, then the image's own proportions win. Existing content is fixed without a migration, because the correction is in rendering.

Verification

Headless Chromium, a 1000x250 image stored as 500x500, real serializer output through DOMPurify with the reader's configuration:

markup	rendered
before, aspect-ratio: 500 / 500	500x500
after, aspect-ratio: auto 500 / 500	500x125

• content-html.test.ts states the reader's promises about image dimensions as V1-V6, with a fast-check property over width and height up to the extension's maximum. Its first run failed at [1, 4097]: the reader trusts stored dimensions only up to 4096 px (safePositiveInt), a pre-existing bound that is now stated as V4 rather than excluded from the generator. The property keeps one unguarded assertion across both branches: never a ratio without auto.
• Coverage of the touched lines: 100 % (the one executable line changed ran 243 times; the rest are comments).
• Stryker over content-html.ts with the suites that reach it: on the image branch 21 killed, 8 survived before this PR's extra tests; both changed lines are killed; three survivors on the branch are killed by two added tests, each verified by hand mutation; the remaining five are equivalent (optional chaining behind the src check, ternaries with identical results). Whole file: 117 killed, 47 survived, 159 no coverage. The file is not in the mutation manifest and this PR does not add it, for the reasons in SELF-IMPROVE.md ("all-or-nothing per file").
• typecheck, oxlint, prettier: green.

What this does not fix

The stored 500x500 defaults are still wrong data. In the browser that is now only a short layout shift while the image loads. Mail clients honour width/height attributes literally, so a changelog or conversation notification email can still show a squashed image. The fix for that is to read the real dimensions at insertion time, at the four insertion points in rich-text-editor.tsx.

Also bumps the SELF-IMPROVE counter on "the coverage and mutation gates read HEAD, not the working tree" to 2x.

🤖 Generated with Claude Code