fix(graph): resolve status-footer stuck-progress, log clobbering, and rewalk loop

[germanamz]

Summary

Fixes the three defects in the tusk graph status footer reported in the issue:

1. Footer read as stuck progress. The footer showed index gen N, the
   reindex_gen walk counter that bumps on every walk — including no-op walks
   — so a quiet, fully-indexed workspace left it frozen at some N that reads as N
   pending items. The footer now names the state (synced / indexing… /
   walk error) and, once a walk has completed, summarizes it
   (last walk 1ms (0 changed)), sourced from a new goroutine-safe WalkStatus
   tracker updated by the walk sites. reindex_gen semantics are unchanged (it
   is load-bearing for generation-based orphan reap); it is simply no longer the
   prominent number on screen.

2. -v logs glued onto the footer. On an interactive TTY the sticky footer
   (\r+erase-line, no trailing newline) and the background slog lines share the
   terminal, so log lines concatenated onto the footer text (…[q] quittime=…).
   A new footerWriter multiplexes the footer and the logs onto one stream,
   erasing and redrawing the footer around each log line, mutex-serialized.

3. Redirect-into-tree rewalk loop. tusk graph -v > graph.log inside the
   workspace armed a permanent ~500ms walk loop: each walk's own log write was a
   MODIFY the watcher turned into the next walk. The watcher now drops events
   whose path can never be a node — anything that is not a node file
   (.md/.html/.htm), the manifest (tusk.toml), or a directory. This also
   fixes the same footgun in tusk watch.

Verification

• Full suite green: go test ./... (37 packages, 0 fail), make vet,
  make lint (0 issues).
• Live end-to-end against the issue's repro workspace with the built binary:
  • Defect 3: after one real change the generation counter went 3→4 and stayed
    at 4; only 2 walks total (was climbing ~2/sec indefinitely).
  • Defect 1 (PTY): footer transitions indexing… → synced · 2 nodes · 2 edges · 0 clients · last walk 1ms (0 changed); no index gen.
  • Defect 2 (PTY, -v): every log line is preceded by an erase-line; no
    [q] quittime= glue.

Notes

• Extensionless files (e.g. Makefile) still trigger a walk — conservative, so
  directory renames/deletes (whose events name the extensionless dir) still
  reindex; no loop risk since the walk never writes those.
• The graph command's background logs now flow through one shared logger over the
  footer coordinator (still stderr); non-interactive runs leave it a transparent
  passthrough.