fix(security): path-traversal guard on 5 sensing-server endpoints (closes #615) (#616)

Reported by @bannned-bit. Five endpoints in
v2/crates/wifi-densepose-sensing-server embedded user-controlled
identifiers in format!() paths with no sanitization:

  recording.rs       POST   /api/v1/recording/start       (session_name)
  recording.rs       GET    /api/v1/recording/download/:id (id)
  recording.rs       DELETE /api/v1/recording/delete/:id   (id)
  model_manager.rs   POST   /api/v1/models/load           (model_id)
  training_api.rs    load_recording_frames                (dataset_ids[])

Each unauthenticated caller could:
- READ arbitrary files via ../../etc/passwd, ../../.env, etc.
- WRITE attacker-controlled JSONL via recording/start
- LOAD attacker-controlled .rvf model files
- DELETE arbitrary files the server process can touch

New `path_safety` module exports `safe_id(&str) -> Result<&str, PathSafetyError>`
that enforces the rejection envelope BEFORE any user input reaches a
format!() that builds a path:

  - Allowed character set: [A-Za-z0-9._-]
  - Reject leading '.' (rules out '.', '..', '.env', hidden files)
  - Reject empty strings
  - Reject anything > 64 bytes
  - Reject all whitespace, path separators, null bytes, non-ASCII

Applied at all 5 sites. Errors return 400 Bad Request (download) /
status:"error" JSON (others) — not panics.

9 unit tests in path_safety::tests cover:
  - accepts simple alphanumeric / hyphen / underscore / dot
  - rejects empty, leading dot, path separators ('/', '\'),
    null byte, whitespace, shell specials, non-ASCII (including
    fullwidth slash U+FF0F), too-long, boundary at MAX_ID_LEN

  test result: ok. 9 passed; 0 failed
  cargo build -p wifi-densepose-sensing-server --no-default-features: 33s

Fix-marker RuView#615 in scripts/fix-markers.json prevents removing the
guard at any of the 5 call sites. CHANGELOG entry under [Unreleased] /
Security documents the patched endpoints and the rejection envelope.

Severity: critical per reporter — five remotely-reachable paths to read,
write, or delete arbitrary files. Hot per-request paths, not edge cases.

scripts/fix-markers.json

@@ -172,6 +172,22 @@
       "rationale": "The per-node TDM/channel overlay intentionally omits WiFi creds (those live in the base flash image). Without --force-partial the issue #391 wifi-trio guard in provision.py rejects the call and breaks the Swarm Test (ADR-062) job. Was red on main for ~5 weeks before PR #590.",
       "ref": "https://github.com/ruvnet/RuView/pull/590"
     },
+    {
+      "id": "RuView#615",
+      "title": "path_safety::safe_id gates user-controlled IDs at filesystem boundaries",
+      "files": [
+        "v2/crates/wifi-densepose-sensing-server/src/path_safety.rs",
+        "v2/crates/wifi-densepose-sensing-server/src/recording.rs",
+        "v2/crates/wifi-densepose-sensing-server/src/model_manager.rs",
+        "v2/crates/wifi-densepose-sensing-server/src/training_api.rs"
+      ],
+      "require": [
+        "path_safety::safe_id",
+        "pub fn safe_id"
+      ],
+      "rationale": "Five endpoints used to embed user-controlled identifiers (session_name, model_id, dataset_id, recording id) into format!() paths with no sanitization, allowing classic '../../etc/passwd' reads, writes, and deletes on the server filesystem. The safe_id helper enforces [A-Za-z0-9._-] only (no leading '.', max 64 chars) and must run before any user input reaches a format!() that builds a path. Removing the helper or skipping it at any of these call sites reintroduces the #615 attack surface.",
+      "ref": "https://github.com/ruvnet/RuView/issues/615"
+    },
     {
       "id": "RuView#560",
       "title": "verify.py quantizes features before SHA-256 for cross-platform hash stability",