Address the open items from the FEATURES.md audit backlog:
- saveChannels() now skips unused slots (all-zero secret) instead of
writing all 40 every time, so /channels2 holds only configured channels
(was always ~2.7 KB) and wears the flash less. loadChannels() already
compacted empties on read, so the loaded result is unchanged.
- findChannelIdx() returns -1 for an all-zero secret, so a corrupted/empty
channel can't match an unused all-zero slot and misroute messages.
- TrailStore gains a static_assert that CAPACITY fits the uint16_t save
header count, failing the build instead of silently truncating.
Also re-classify two audit items verified to be non-issues in current
code: the bot strstr "199-char" truncation (BOT_SCRATCH=200 >= MAX_TEXT_LEN
=160, unreachable) and the MSG_PICK reply title (rlen clamped to 20, so
title[24] never overflows).
Verified: WioTrackerL1_companion_solo_dual builds clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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