feat: add SVN (Subversion) working copy support

[xiaoye5200]

Summary

Adds first-class SVN support alongside the existing Git integration,
so that projects using Subversion can build and incrementally update
the knowledge graph without any manual configuration.

What changed

incremental.py

• find_svn_root() — walks up the directory tree to locate the SVN
  working-copy root (handles both SVN 1.6 per-directory .svn and
  the single-root layout introduced in SVN 1.7).
• detect_vcs(root) — returns "git", "svn", or "none" based on
  VCS markers at a given path. Used internally to dispatch to the
  correct VCS backend.
• find_repo_root() — now falls back to find_svn_root() when no
  .git directory is found.
• get_changed_files() — for SVN repos, uses svn status (working-
  copy changes) or svn diff --summarize -r rXXX:HEAD when the base
  parameter is a valid SVN revision range.
• get_staged_and_unstaged() — delegates to svn status for SVN repos.
• get_all_tracked_files() — tries svn list -R for SVN repos, falls
  back to a filesystem walk (which is also the existing Git fallback).
• _svn_revision_info() — reads branch path and revision number from
  svn info.
• _store_vcs_metadata() — helper that writes either
  git_branch/git_head_sha or svn_branch/svn_revision metadata
  depending on the detected VCS; used in both full_build() and
  incremental_update().
• Added .svn/** to DEFAULT_IGNORE_PATTERNS.

changes.py

• parse_svn_diff_ranges() — runs svn diff and feeds the unified-diff
  output into the existing _parse_unified_diff() for line-level change
  mapping. Accepts an optional revision range.
• parse_diff_ranges() — VCS-aware dispatcher: routes to the SVN or
  Git diff parser based on the presence of a .svn marker.
• analyze_changes() now calls parse_diff_ranges() instead of
  parse_git_diff_ranges(), so line-level risk scoring works for SVN
  diffs automatically.

tools/review.py — get_review_context uses parse_diff_ranges()
instead of parse_git_diff_ranges().

tools/__init__.py — exports parse_svn_diff_ranges and
parse_diff_ranges alongside the existing parse_git_diff_ranges for
backward compatibility.

cli.py — the status command shows SVN branch and revision number
for SVN working copies instead of the Git branch/SHA block.

Backward compatibility

All existing Git behavior is unchanged. The SVN path is only activated
when a .svn directory is detected; every function still accepts the
same parameters as before.

Testing

• Verified with a local SVN working copy (SVN 1.14, server-side).
• All existing test_incremental.py and test_changes.py tests pass
  without modification.
• Pre-existing Windows teardown failures (file-lock on SQLite temp
  files in teardown) are unrelated to this PR.

Notes

• svn list -R queries the server and may be slow on large repos;
  the filesystem-walk fallback kicks in automatically if it fails or
  times out.
• The base parameter semantics for SVN: a string matching
  r?\d+(:r?\d+|:HEAD|:BASE|:COMMITTED)? is treated as an SVN
  revision range; anything else (including the default "HEAD~1")
  is ignored and svn status is used instead.