ESP32-S3 bloat gate red on master: a fresh fbuild build of Blink now links Arduino's Matter library (1.6 MB)

## Observed
The ESP32-S3 bloat gate's Blink build went from 462 KB to 1.6 MB with no change on our side:

| run | commit | framework core key | core files | image |
|---|---|---|---:|---:|
| master bloat run 35436746934, attempt 1 (2026-09-19 10:12Z) | `8999762397` | `07cf2eef0eca` | 59 | 462.49 KB → PASS |
| **same run, attempt 2** (re-run 17:2xZ) | `8999762397` | `85bbccbc1fa1` | 60 | **1.60 MB → FAIL** (gate total 1,456,263 B vs 383,158) |
| PR #4473 (twice) | PR merge ref | `85bbccbc1fa1` | 60 | 1.60 MB → FAIL |

The same commit, re-run, flips. Both attempts install `fbuild==2.5.24` and `zccache==1.14.3`. Everything `fbuild install --dry-run` resolves for esp32s3 is pinned: platform `54.03.20`, arduino-esp32 `3.3.11` core and libs, toolchain `14.2.0_20260121`. It also lists a second platform entry from the floating `releases/download/stable/` URL.

## What gets linked
Diffing the failing report (bloat artifact, #4470) against a passing one:

| archive | + flash |
|---|---:|
| `libespressif__esp_matter.a` | 334 KB |
| `libnet80211.a` | 119 KB |
| `liblwip.a` | 96 KB |
| `libbt.a` / `libbtdm_app.a` | 93 KB / 79 KB |
| `libmbedcrypto.a`, `libwpa_supplicant.a`, `libpp.a`, `libespressif__mdns.a`, `libphy.a` | ~240 KB |

The chain starts in Arduino's `Matter` library (`libmatter.a`, e.g. `MatterWiFiStackWrap`). Its references pull in esp-matter, Wi-Fi and BT. So a fresh build's library selection (`lib_ldf_mode = chain`) now picks the `Matter` library for Blink.

## Impact
Every PR touching `src/**` fails the bloat gate, e.g. #4473, and master's next push run will too. Builds with a warm local fbuild cache don't reproduce it: locally, Blink is still 473,524 B.

## Not done
The baseline is **not** re-pinned. 1.6 MB is a wrong build, not a cost. The fix belongs to fbuild's library selection, or to whatever it now reads that it didn't at 10:12Z (the floating `stable` platform is the only unpinned input visible). Filing the fbuild side next.

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

[]