chore: brutal comment audit — obliterate restatement, narration, decoration

[3lvis]

Ran the comment-audit bar at full aggression across the whole first-party tree — doc comments included — and stripped everything the code already says.

Net: −336 comment lines (479 deleted / 143 kept-or-reflowed, 47 files).

What died

• Signature-restating docstrings (///), public or not — @SyncQuery/@SyncModel/publisher-family navigation docs, init/getter docs that re-say the name.
• Narration & step labels — // insert, // Reconnect:, // Act/// given-when-then, assertion restatement (the test names + messages carry it).
• Decoration — every // MARK: (22 of them).
• Snapshot rot & duplicated rationale — counts, "currently", and whys repeated at call sites.

What survived (the narrow bar)

Only what the code genuinely can't express, trimmed to the load-bearing clause:

• external / wire-protocol facts (server LWW, upsert-by-id, payload absent vs null semantics);
• non-obvious whys (token-advances-only-on-clean-pass, capture-before-upload, per-pass inbox annotation);
• framework gotchas (SwiftData history .preserveValueOnDeletion, @unchecked Sendable soundness, nil != "inbound" is SQL NULL);
• justifications for suspicious code (force-casts, empty closures, force-unwraps that are intentional);
• the repo-required UI-test "why this costly test earns its place" rationales.

Verification

Behavior-neutral (comment-only + reflow). SwiftSync 190, DemoCore 44, DemoBackend 32, demo app builds, DemoUITests 3/3 — all green.