docs(pptx): say what a partial capability actually costs

[DemchaAV]

Why

render-pptx/README.md is the page GitHub renders for the artifact, and it carried the same overstatement the release notes did until #444: all ten partial capabilities described as rendering "natively with an approximated styling detail". Reading the matrix rows rather than the count shows only some are that. The others lose content the format cannot carry.

The opening sentence also lumped "links and navigation" together as native. Links are — hyperlinks and internal slide jumps render as real PowerPoint links. Bookmarks are the degraded half: PPTX has no outline tree, so the first bookmark on a page names its slide and the rest are dropped. A reader with a bookmark-heavy document was told the wrong thing about the one part that actually loses data.

What changed

• The capability summary splits the partial group instead of characterising it as one thing.
• Three content-losing cases added to Limitations to know before you ship: dropped bookmarks, the inline-SVG PNG fallback, and the unrepresentable metadata producer field.
• The clip raster's resolution behaviour is recorded — it targets a 2048-pixel long edge clamped between native size and 4×, so a larger region renders at native resolution rather than downscaled, at a transient memory cost proportional to its size. The capability matrix has carried this since the clamp landed in #442; this page did not.

Every other claim on the page re-verified

Claim	Result
not pulled in by graph-compose / -core / -bundle	correct — bundle/pom.xml depends on wrapper + templates + fonts + emoji only
render-pdf at compile scope	correct — no <scope> element
SLIDE_16_9 960 × 540, SLIDE_4_3 720 × 540	correct (DocumentPageSize:35/43)
renderToImages throws	correct
determinism is opt-in	correct
PptxSemanticBackend footnote	the class is still shipped
install pin 2.0.0	correct pre-cut state — cut-release.ps1 flips it, VersionConsistencyGuardTest guards it
both doc links + migration guide	resolve on disk

Verification

./mvnw -B -ntp clean verify — BUILD SUCCESS, 1522 tests, 0 failures.