Fix edge-case bugs found in the 3.1 review

[bbatsov]

A careful review of the recent v3.1 work (sessions, watches, async scan, reviewable replace/search, frecency, file kinds) turned up mostly solid code with a handful of real edge-case bugs. This fixes the six worth acting on, each with a regression test.

• projectile-toggle-related-file breaks under a symlinked project root - it hand-rolled the relative path and didn't file-truename, so you'd get a spurious "No related files found". Same class as the frecency fix in 196e45c; routed through the documented projectile--project-relative-name helper.
• Replace/Search help advertised the wrong quit key - \[quit-window] rendered as "C-x w q quit" instead of the actual q. Now \[projectile-replace--quit] in both help lines.
• projectile-purge-file/dir-from-cache leaked watches - they mutated the cache without re-deriving the file-notify watches, so watches stayed on the purged path and a later create-event re-added the just-purged entries. Now call projectile--maybe-watch-project on the shrunken cache, like projectile-cache-project.
• Session restore could block startup on a prompt - find-file-noselect can fire a large-file warning or an unsafe-file-locals prompt, which the surrounding ignore-errors won't suppress. Restore now visits files non-interactively.
• A corrupt :window-state aborted the whole restore - now caught (matching the buffer-recreation guard right above it), so the recreated buffers are kept.
• A stale async-scan chunk could bleed into a fresh scan - the scan now carries a generation token; a timer that fires after its scan was superseded/canceled no-ops instead of appending to a newer scan's match list.

Nice-to-haves, doc drifts, the version-metadata 3.1.0-vs-3.2.0 inconsistency, and the suggested test-coverage gaps (rg-vs-elisp parity, etc.) are left for follow-ups. 882 specs green, clean --warnings-as-errors, relint clean.