fix(firmware): address PR #397 review feedback

Applies @ruvnet's five review requests on PR #397 (RuView#397 comment
4289417527):

1. **Inline comment on `provision.py` `write_flash`** — ESP-IDF v5.4
   bundles esptool 4.10.0 (underscore-only). #391's hyphen swap broke
   the documented venv flow; kept the underscore form and added a
   three-line comment warning future maintainers not to "re-fix" it.

2. **Correct `edge_processing.c` sample_rate** (blocking) — changed
   hard-coded `20.0f` → `10.0f` at line 718 so
   `estimate_bpm_zero_crossing()` matches the MGMT-only CSI rate.
   Without this, breathing and heart-rate reports were 2× the true
   value. Added a comment tying the constant to the callback rate gate.

3. **Removed disabled probe-injection infrastructure** — dropped the
   forward declaration, the `CSI_PROBE_INTERVAL_MS` define, six static
   variables (`s_probe_timer`, `s_probe_tx_count`, `s_probe_tx_fail`,
   `s_ap_bssid`, `s_ap_bssid_known`), and three functions
   (`csi_send_probe_request`, `probe_timer_cb`,
   `csi_collector_start_probe_timer`). None were reachable.
   `csi_inject_ndp_frame()` reverted to the original ADR-029 stub.
   Can be revived from this commit's parent if needed.

4. **Cleaned `sdkconfig.defaults`** — removed the SPIRAM prose and
   commented-out `# CONFIG_SPIRAM is not set` line. Kept only the live
   `CONFIG_ESP_WIFI_EXTRA_IRAM_OPT=y` with a concise rationale.

5. **Bumped firmware version 0.6.1 → 0.6.2** and added four
   `[Unreleased]` CHANGELOG entries covering the SPI cache crash fix,
   the `filter_mac` / `node_id` clobber defense, the sample-rate
   correction, and the `write_flash` command-form revert.

Net: +39 / -128 across six files.

Validation in this devcontainer:
- Static sanity on modified C files: braces balance (csi_collector.c
  59/59; edge_processing.c 96/96), zero dangling references to removed
  probe-injection symbols.
- Rust workspace tests and Python proof not executed here — cargo not
  installed and pip blocked by PEP 668. Deferring hardware build +
  flash + miniterm verification to @ruvnet's COM7 per his offer in
  the review comment.

Co-Authored-By: claude-flow <ruv@ruv.net>

firmware/esp32-csi-node/provision.py

@@ -155,6 +155,9 @@ def flash_nvs(port, baud, nvs_bin):
             "--chip", "esp32s3",
             "--port", port,
             "--baud", str(baud),
+            # Keep underscore form — ESP-IDF v5.4 bundles esptool 4.10.0 which only
+            # accepts "write_flash". pip's esptool >=5.x accepts both (hyphenated
+            # form preferred) but keeps underscore working. Do not "correct" this.
             "write_flash",
             hex(NVS_PARTITION_OFFSET), bin_path,
         ]