[eas-cli] Skip rewriting unchanged screenshots in metadata:pull

[ahmdshrif]

Why

Fixes #3804.

Running eas metadata:pull marks every downloaded screenshot as changed on each invocation, even when nothing about the image changed. App Store Connect embeds a unique asset resource ID into each screenshot's eXIf (UserComment) and iTXt (Adobe XMP) PNG chunks on every download, so the downloaded bytes always differ from the local file — while the decoded image (IDAT) is byte-for-byte identical. downloadScreenshotAsync writes the response buffer unconditionally, so every pull dirties version control with meaningless diffs.

How

• Added isPngEquivalent(a, b) in metadata/utils/png.ts: walks both buffers' PNG chunk sequences and compares them ignoring the volatile metadata chunks (tEXt, zTXt, iTXt, eXIf, tIME). All other chunks — including all pixel data — must match exactly. Falls back to plain byte equality when either buffer is not a structurally valid PNG, so non-PNG assets keep today's behavior.
• In downloadScreenshotAsync, when a local file already exists at the output path, the write is skipped if the existing file is PNG-equivalent to the downloaded buffer (logged as Screenshot unchanged). The config entry still points at the same relative path, so metadata.json/sync behavior is unchanged.

No new dependencies — the chunk walk is ~30 lines over the documented PNG structure (8-byte signature + length/type/data/CRC chunks).

Test Plan

• yarn test src/metadata — 16 suites, 195 tests pass, including:
  • new unit tests for isPngEquivalent (metadata/utils/__tests__/png.test.ts): equivalent when only iTXt/eXIf/tEXt differ, not equivalent when IDAT/PLTE differ, byte-equality fallback for non-PNG buffers, truncated-PNG safety;
  • new task test (screenshots.test.ts): a pull where the local file differs from the download only in the iTXt chunk does not call fs.promises.writeFile, and still writes the config entry.
• yarn lint, yarn fmt:check, yarn typecheck — clean (remaining warnings/errors are pre-existing on main and untouched by this PR).
• Manual reproduction: run eas metadata:pull twice in a git-tracked project with existing screenshots — before this change every screenshot shows as modified on the second pull; after, git status stays clean.

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