CI: track RAM (.data+.bss) in size reports and gate on per-env budgets (#10899)

* CI: track RAM (.data+.bss) in size reports and gate on per-env budgets

The 2.8.0 nRF52840 heap regression (99% heap in field reports) shipped
invisibly because CI only tracked flash. On nRF52840 the heap arena is the
linker gap after .bss, so every byte of static .data+.bss growth shrinks the
usable heap 1:1 - RAM needs the same guardrails flash already has.

What's added:

- bin/platformio-custom.py emits ram_bytes (.data + .bss from the ELF, via
  the toolchain size tool) into each .mt.json manifest. Heap/stack
  placeholder sections are deliberately excluded.
- bin/collect_sizes.py records {flash_bytes, ram_bytes} per env;
  bin/size_report.py grows RAM and RAM-delta columns in the PR size report.
  Older artifacts without ram_bytes (and legacy int-schema baselines)
  degrade to "n/a" instead of crashing.
- bin/ram_budgets.json: per-env RAM/flash budgets, enforced only for envs
  listed there. Seeded with rak4631: ram 113,000 (current 110,948 + ~2 KB
  slack), flash 786,000 (current 765,192 + ~20 KB; the app region is
  0x27000..0xEA000 = 798,720 and the image must stay clear of the
  warm-store ring guard in extra_scripts/nrf52_warm_region.py).
- New size-budget-gate CI job runs size_report.py --enforce-budgets and
  fails the build on violation; the informational firmware-size-report job
  now also renders budget usage into the PR comment.
- src/main.cpp: opt-in boot heap watermark (-DMESHTASTIC_HEAP_WATERMARK_CHECK)
  logs LOG_ERROR when less than 20% of the heap is free at the end of
  setup(). Off by default; skipped on platforms without heap accounting.

How budgets are raised: deliberately, never automatically. If a change needs
more headroom, bump the env's limit in bin/ram_budgets.json in the same PR
and justify the increase in the PR description.

Verified: python3 bin/test_size_scripts.py (23/23 pass, including ram_bytes
parsing, n/a fallback, and over/under/missing-env budget-gate cases).

* Address review: fail the budget gate closed, fix RAM section matching

- size-budget-gate workflow: drop continue-on-error on the manifest
  download and the empty-dir fallback, so the job fails when the data it
  gates on cannot be fetched.
- size_report.py --enforce-budgets now fails closed on every
  missing-data path instead of trivially passing: empty collected sizes,
  a budgeted env that was not built, or a manifest without the budgeted
  metric. Report-only mode keeps rendering those as n/a.
- load_budgets() rejects zero/negative/non-integer budgets with a clear
  error (a typo'd budget could previously crash budget_markdown with
  ZeroDivisionError or silently skip the check); the percentage render
  keeps a defensive guard for direct callers.
- compute_ram_bytes(): count RISC-V small-data sections (.sdata/.sbss)
  and exclude ESP-IDF .rtc.* sections, which live outside the
  heap-competing SRAM.
- Trim the heap-watermark comment in main.cpp to two lines.

bin/test_size_scripts.py: 27/27 - the two fail-open assertions are
flipped to fail-closed, with new report-only counterparts plus cases for
empty sizes under enforcement and malformed budgets.

bin/collect_sizes.py

@@ -1,6 +1,15 @@
 #!/usr/bin/env python3
 
-"""Collect firmware binary sizes from manifest (.mt.json) files into a single report."""
+"""Collect firmware binary sizes from manifest (.mt.json) files into a single report.
+
+Output schema (consumed by bin/size_report.py):
+    {"<env>": {"flash_bytes": <int>, "ram_bytes": <int>}}
+
+flash_bytes is the size of the main firmware image (.bin). ram_bytes is the
+static RAM footprint (.data + .bss) emitted into the manifest by
+bin/platformio-custom.py; it is omitted for manifests that predate it, and
+size_report.py renders those as "n/a".
+"""
 
 import json
 import os
@@ -8,7 +17,7 @@ import sys
 
 
 def collect_sizes(manifest_dir):
-    """Scan manifest_dir for .mt.json files and return {board: size_bytes} dict."""
+    """Scan manifest_dir for .mt.json files and return {board: sizes_dict}."""
     sizes = {}
     for fname in sorted(os.listdir(manifest_dir)):
         if not fname.endswith(".mt.json"):
@@ -34,7 +43,11 @@ def collect_sizes(manifest_dir):
                     bin_size = entry["bytes"]
                     break
         if bin_size is not None:
-            sizes[board] = bin_size
+            entry = {"flash_bytes": bin_size}
+            ram_bytes = data.get("ram_bytes")
+            if isinstance(ram_bytes, int) and not isinstance(ram_bytes, bool):
+                entry["ram_bytes"] = ram_bytes
+            sizes[board] = entry
     return sizes