feat: ASCII animation player + anim%save fix + gfortran 16 spec workaround

[krystophny]

Summary

• New `fortplot_ascii_player` module + `fortplot_play_ascii` CLI that play back `.txt` animations produced by `save_animation('*.txt')`. Parses the `=== Frame N ===` framing the existing pipeline already emits, prints frames to stdout (or any unit), and sleeps via libc `nanosleep` between frames.
• Make `anim%save(...)` work directly as the README documents. Previously the type-bound save returned -1 unless `save_animation()` had been called first to register the impl pointer; now the facade `FuncAnimation` wires it up at construction.
• Reconcile the README with reality: animation save accepts `.txt` (no ffmpeg needed); add a player invocation snippet.
• Fix an unrelated gfortran 16 `-O0` miscompile that crashed `test_spec` on system gfortran 16.1.1. Reduced and filed upstream as [gfortran 16, -O0] deferred-length character concatenation produces all-blanks for certain RHS shapes (64-char total) krystophny/gcc-dev#167. Worked around in `spec_to_json` by routing the schema header through a `len=*` formal.

Test plan

• `fpm test test_ascii_player` -- parse-header edge cases, frame count, dry-run, capture-to-unit, missing-file / zero-fps error paths.
• `fpm test test_ascii_player_e2e` -- save_animation .txt -> player end-to-end.
• `fpm test test_animation_txt_save` -- existing .txt animation save test still green.
• `make example ARGS="save_animation_demo"` -- mp4 save still works.
• `make test` -- whole suite green on local gfortran 16.1.1 (CachyOS / Arch).

Verification

Test fails on main (system gfortran 16, -O0)

```
$ git checkout main && fpm test test_spec
...
FAIL: rt line: parse succeeds
FAIL: rt line: mark type
...
At line 669 of file test/test_spec.f90
Fortran runtime error: Index '1' of dimension 1 of array 'parsed...%columns' below lower bound of
Error termination.
cmd_run:stopping due to failed executions
STOP 2
```

```
$ git checkout main && fpm test test_animation_txt_save # writes a .txt animation

(passes, but no way to play it back -- before this PR)

```

```
$ # README example pattern: anim%save("movie.txt", status=status)

Returns -1 with "Animation save implementation not initialized" before this PR.

```

Test passes after fix

```
$ make test 2>&1 | tail -3
ALL TESTS PASSED (fpm test)
```

```
$ fpm test test_spec 2>&1 | tail -4
=== Spec Test Summary ===
Tests passed: 158 / 158
All spec tests PASSED!
STOP 0
```

```
$ fpm test test_ascii_player test_ascii_player_e2e 2>&1 | tail -4
PASS test_ascii_player
PASS test_ascii_player_e2e
```

Player demo

```
$ fpm run --target fortplot_play_ascii -- /tmp/wave.txt --fps 24 --loop
=== Frame 1 ===
ascii animation
+--------------------------------------------------------------------------------+
|...|
... (animated)
```

Notes

• Out of scope: 3D ASCII rendering + 3D animation parity across backends (next PR).
• Out of scope: optimize the throughput of `save_ascii_animation` (separate follow-up PR).
• Companion compiler issue: [gfortran 16, -O0] deferred-length character concatenation produces all-blanks for certain RHS shapes (64-char total) krystophny/gcc-dev#167.