fix(tr): surface firmware NAK text on launch/upload failures

[kfox]

What

When the TeensyROM NAKs a command, teensyrom_dma._expect_ack drained the firmware's trailing error text only to resync the stream, then discarded it — so a failed PostFile/LaunchFile surfaced just a bare FailToken (0x9B7F). This captures that text and puts it in the raised error:

• "Busy!" (a program is running / the menu handler isn't active) → new TRBusyError (subclass of TRError), so callers can distinguish it from a hard failure.
• Other reasons ("Not enough room", "File already exists.", …) → appended to the FailToken/RetryToken/unexpected-reply message.

The drain (resync) behavior is unchanged — we just stop throwing the reason away. Covered by ErrorSurfacingTest against the loopback transport. No launch-path behavior change.

Why (context)

This is the safe, salvaged part of a TR-launcher investigation. The original goal (reset-free relaunch for looping launchers) turned out not verifiable and was backed out. The investigation also uncovered a separate, pre-existing intermittent bug: the keyboard poller's ReadC64Mem racing the launcher's reset()+PostFile on the shared TR link can desync the stream so the upload drops a byte — the .prg then loads one byte short and the BASIC autostart stub ?SYNTAX ERRORs. It's a race (reliable single-threaded, and on the Ultimate which has no shared-link poll), so verifying a fix needs a soak harness; it's tracked as a known issue in docs/caveats.md, not fixed here. This error surfacing is what makes that failure diagnosable.

Test

make check (ruff + mypy --strict + pyright + 1588 tests) green.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.69%. Comparing base (1ad2588) to head (b8801a9).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
c64cast/teensyrom_dma.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   79.68%   79.69%           
=======================================
  Files          68       68           
  Lines       12943    12947    +4     
  Branches     1910     1911    +1     
=======================================
+ Hits        10314    10318    +4     
  Misses       2190     2190           
  Partials      439      439           

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