feat(core): filesystem timestamp-granularity probe with ctime fallback and per-directory gitignore cascade (#141)

Implements DESIGN s5.2 step 2 end-to-end plus the true per-directory
gitignore cascade (both prior `TODO(M3)`s in the scanner/exclude
modules).

## Timestamp-granularity probe + ctime fallback
- On the first scan of a source, a write-stat probe measures the
filesystem's effective mtime granularity (bounded by a ~2.2s budget so
even 2s-resolution FAT32 resolves in one probe). The result is persisted
per-source (migration `0009`); later scans reuse it. A probe I/O failure
degrades to "fine" for that cycle only and re-probes next scan.
- Change detection stays `(size, mtime_ns)` on fine filesystems (zero
extra cost). On a coarse FS (granularity > 1s), a stat-matched file is
re-hashed when its ctime/inode-birth post-dates the last scan boundary
or its mtime falls within one granularity window of it - catching
within-quantum in-place edits that a pure mtime compare misses.
- Decision logic is pure (`granularity_is_coarse`,
`coarse_fs_needs_rehash`) and unit-tested across the rule branches.

## Per-directory gitignore cascade
- Replaces the flattened single-matcher approximation with
directory-scoped cascading scopes: nested `.gitignore`/`.ignore` files
apply at their own depth and deeper files override shallower ones,
preserving negation/re-include semantics.
- The repo-level `.git/info/exclude` and global-gitignore tiers are
anchored at the source root per git semantics (second commit fixes the
tier being scoped to `.git/info/`, which made its rules match nothing).

Verified: 310 driven-core lib tests green, fmt clean, clippy `-D
warnings` clean workspace-wide, `cargo check --workspace` clean, `.sqlx`
regenerated for the migration.

Refs #34

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01QZQVP2tUuTLh8oL31D8heC

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: pmaxhogan

.sqlx/query-68f85a137d75f3bb06d7c864a2b05aa0c6225d41c931595997629946b1d51d28.json

@@ -147,6 +147,7 @@ fn source_in(account: AccountId, root: &std::path::Path, folder_id: &str) -> Sou
         deep_verify_interval_secs: 604_800,
         last_full_scan_at: None,
         last_deep_verify_at: Some(0),
+        mtime_granularity_ns: None,
         created_at: 0,
     }
 }

.sqlx/query-86a9b9886667d72dfe7576dc54652d01d50da00987fdca31f12cdd37c79efcd8.json

@@ -103,6 +103,7 @@ fn source_in(account: AccountId, root: &std::path::Path, folder_id: &str) -> Sou
         deep_verify_interval_secs: 604_800,
         last_full_scan_at: None,
         last_deep_verify_at: Some(0),
+        mtime_granularity_ns: None,
         created_at: 0,
     }
 }

.sqlx/query-b38108b7bb9152533f8f85dcdfc45ec4d150f324ce9a22486a949a0fc062289a.json

@@ -151,6 +151,7 @@ fn source_in(account: AccountId, root: &Path, folder_id: &str) -> SourceRow {
         deep_verify_interval_secs: 604_800,
         last_full_scan_at: None,
         last_deep_verify_at: Some(0),
+        mtime_granularity_ns: None,
         created_at: 0,
     }
 }

…ff94afa9ecbcc80860482a5e6369f3a86df.json …2fde08a50c3d2d439ac18f3a1dac57ce103.json.sqlx/query-7ab8e9edaf83ed2697ef04c2249b7ff94afa9ecbcc80860482a5e6369f3a86df.json renamed to .sqlx/query-ca39a4f265b2a6b40b93de4111e6a2fde08a50c3d2d439ac18f3a1dac57ce103.json .sqlx/query-7ab8e9edaf83ed2697ef04c2249b7ff94afa9ecbcc80860482a5e6369f3a86df.json renamed to .sqlx/query-ca39a4f265b2a6b40b93de4111e6a2fde08a50c3d2d439ac18f3a1dac57ce103.json

@@ -249,6 +249,7 @@ fn source_row(account: driven_core::types::AccountId, root: &Path, folder_id: &s
         deep_verify_interval_secs: 604_800,
         last_full_scan_at: None,
         last_deep_verify_at: Some(0),
+        mtime_granularity_ns: None,
         created_at: 0,
     }
 }

…22a1ac0bac4ea492559ec6229af2e94fe32.json …648ee602223132db5d30b36a7b5a5967588.json.sqlx/query-ea24f36f62c84328655dbf258eb7422a1ac0bac4ea492559ec6229af2e94fe32.json renamed to .sqlx/query-f0e8817b58a17e8596b11d5b1781f648ee602223132db5d30b36a7b5a5967588.json .sqlx/query-ea24f36f62c84328655dbf258eb7422a1ac0bac4ea492559ec6229af2e94fe32.json renamed to .sqlx/query-f0e8817b58a17e8596b11d5b1781f648ee602223132db5d30b36a7b5a5967588.json

@@ -197,6 +197,7 @@ fn source_in(account: driven_core::types::AccountId, root: &Path, folder_id: &st
         deep_verify_interval_secs: 604_800,
         last_full_scan_at: None,
         last_deep_verify_at: Some(0),
+        mtime_granularity_ns: None,
         created_at: 0,
     }
 }