Correct the docs the code had drifted away from

Applies the audit's sections B, C and D, plus two decisions.

Behaviour (A1): fm_ex1..3 carried stock's `extruder: extruder`, so a clog on
T1..T3 was measured against T0's filament movement. Each motion sensor now
watches its own extruder.

Gate count (A6): "four gates" was the number of make test-* targets, not of
gates run -- run-tests.py emits twelve. The number is dropped rather than
restated, in docs/testing.md and the Makefile help.

B, install instructions that failed as written: STOCK_TGZ_CREATOR5 missing its
.tgz; a pointer to a ./build.sh that does not exist; klippy-extras/ paths that
moved to payload/klipper/extras/ in 0ef9d6b; a calibration recipe both
commands refuse without PLATE_REMOVED=1; a storage table that put dock_x/dock_y
in an include, which SAVE_CONFIG refuses; four files telling the user to add
includes printer.base.cfg already ships; ./pack.sh style paths that do not
resolve from where the scripts leave you.

The MAINSAIL_ZIP / HELIX_TGZ override was documented in three places as "used
as-is, never checksummed". It is checksummed, and fetch-assets.sh overwrites
the user's file with the pinned release -- SKIP re-downloads too. Corrected,
with the way that actually works (match the hash in versions.env).

C, files that contradicted themselves: the -5/-3 ESTOP target, the
t<n>_offset_* storage names, the chamber heater's model split, the LED
premise, the include order comment in test_includes.py, gear_stepper as a
"feed hub" when it is the tool lock, an object list ff-toolchange.cfg says
does not exist, the eddy probe vs the under-bed cylinder, "installs them
without clobbering" against a gate that enforces the clobber, "keeps two
versions" when the installer keeps one, "two changes from stock" when there
are three, and ff_legacy's "nothing here runs during normal operation" when
auto_import runs on every ready.

D, dangling references: OKF/* and firmwareExe-decompiled/* repointed at
docs/notes/*, plus case-boot.sh, sim-image.sh, printer-exec.sh, the
test/printer build context, wait_for_banner, ff_toolchanger_compat and a
nonexistent orca snippet.

Not touched, pending a decision: sections E-H, and the printer.base.cfg
"verbatim" claim (A7), which depends on how the unpack drift check is fixed.

48 passed, 5 skipped.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/how-it-works.md

@@ -48,8 +48,9 @@ S80ui        decides whether the UI runs; owns SAFE-MODE
 
 Everything the mod installs lives under `/usr/data/anvil` on the **data**
 partition, which a FlashForge OTA cannot delete. The software component itself
-goes to `/usr/prog` on the firmware partition, which only holds two versions
-and cannot fit ~100MB of web UI — that is why Mainsail and HelixScreen ride in
+goes to `/usr/prog` on the firmware partition, which keeps only one version
+(the installer wipes every other version directory) and cannot fit ~100MB of
+web UI — that is why Mainsail and HelixScreen ride in
 the outer package instead.
 
 ## Moonraker