feat: preview results found so far during a scan (Tab)

[ultranity]

Problem

During a scan there is currently no way to look at what has been found — you
have to wait for the whole scan to finish. For very large / slow targets that
can be a long time (see #150, where users describe 53–60 TB scans).

Change

Pressing Tab during a scan opens a read-only, point-in-time view of the
directory tree discovered so far, with real aggregated directory sizes. Pressing
Tab (or Esc) returns to the progress modal. The scan keeps running in
the background the whole time.

• The preview does not auto-refresh — re-entering it takes a fresh snapshot
  (so sizes/sorting do not jump around while you read).
• Navigation (arrows / hjkl / Enter) and sorting work; destructive or external
  actions are ignored while previewing, since the tree is still being built.
• The scanning modal shows a Press Tab to preview results found so far hint.

Relation to #150

#150 asks to stop a scan and keep the results (to save RAM / time). This PR is
a related step — you can now inspect partial results during a scan — but it
intentionally keeps the scan running rather than stopping it. Happy to extend
this toward fully resolving #150 (stop-and-keep) if you would like.

Implementation

• analyze: expose the in-progress root via BaseAnalyzer.GetCurrentDir. The
  TUI reads it through an optional interface assertion, so the
  common.Analyzer interface and its other implementations are untouched.
• analyze: Dir.AddFile now takes the write lock and Dir.updateStats reads
  the file list under the read lock, so computing stats on the live tree while
  the analyzer is still appending is race-free (paired with the existing
  RLock in showDir). Verified with a dedicated concurrent -race test
  (TestPreviewWhileScanning).
• tui: enter/exit preview, snapshot stats with a throwaway hard-link map, and
  preview-only key handling.

All tests pass under go test -race ./....

Companion PR #593 adds a confirmation before quitting when a long scan would be lost.

[codecov]

Codecov Report

❌ Patch coverage is 80.61224% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.45%. Comparing base (ee9e4e2) to head (52bfab4).

Files with missing lines	Patch %	Lines
tui/keys.go	73.01%	14 Missing and 3 partials ⚠️
tui/progress.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
- Coverage   85.53%   85.45%   -0.09%     
==========================================
  Files          55       55              
  Lines        6085     6175      +90     
==========================================
+ Hits         5205     5277      +72     
- Misses        676      691      +15     
- Partials      204      207       +3     

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[dundee]

Nice feature, thanks!