* fix(diffs): share SSR preloads and repair language-pack hydration
Render viewer and file documents from a single @pierre/diffs SSR preload
per file (mode=both previously ran the full diff+highlight pipeline
twice; 651ms -> 303ms on an 8-file patch), apply the file-mode font bump
as a document-level override, and keep hydration payloads
variant-faithful.
Fix the language-pack runtime downgrading pack-only languages to plain
text at hydration by defining a per-target build flag and forwarding it
to payload normalization.
Also: case-insensitive language hints, identical before/after
short-circuit with details.changed, patch input failures classified as
tool input errors, canonical config values now win over deprecated
aliases, hash-pinned viewer runtime served immutable, truthful
browser-vs-render errors, timing-safe artifact token compare, unref
idle browser timer.
* docs(changelog): link diffs rendering entry to PR
* test(diffs): narrow manifest validation results before value access
* test(tooling): allowlist diffs viewer-client define suppression
