chore(build): stop compiling the unused flatc translation units

[Naruto]

Description

Both build paths compiled four FlatBuffers translation units that nothing in this project uses:

# ss_player/sources.py
fb_sources = ["idl_parser.cpp", "idl_gen_text.cpp", "reflection.cpp", "util.cpp"]

These are the flatc (schema compiler) side of FlatBuffers. Playback needs none of it — schema compilation is libssconverter's job on the Rust side. They look like a leftover from when .sspj → .ssab conversion was going to happen in C++.

FlatBuffers is used header-only here: the generated format/*.h headers include flatbuffers/flatbuffers.h, whose reachable set (16 headers: buffer/table/vector/verifier/builder…) is entirely inline.

Evidence that nothing linked against them:

• No source includes idl.h, reflection.h or util.h — the only FlatBuffers include anywhere in the project is flatbuffers/flatbuffers.h, via format/*.h.
• Every FlatBuffers symbol used is header-only: Offset, Vector, FlatBufferBuilder, Table, Verifier, GetRoot, GetSizePrefixedRoot, BufferHasIdentifier, EndianScalar.
• nm -u over all 18 built objects (1:1 with the 18 .cpp files) reports zero undefined flatbuffers:: symbols. The linker never asked for anything these four files define.

Changes

• ss_player/sources.py — remove get_fb_sources(), replaced by a comment recording why FlatBuffers is header-only here. Also drop flatbuffers/src from the include path; it was only needed so those translation units could find their own private headers (format/*.h resolves fine from flatbuffers/include alone).
• ss_player/SCsub — remove the add_source_files loop (custom module).
• SConstruct — remove the sources_list.extend(...) call (GDExtension).

Impact

~757 KB of object code per target is no longer built, on every platform/arch a release build covers:

object	size (-O2)
idl_parser.o	574 KB
idl_gen_text.o	84 KB
reflection.o	70 KB
util.o	30 KB

Stale objects from earlier builds confirmed this was being paid across 7 targets already (macOS editor, 3 Android ABIs, 3 iOS slices) — 28 objects, 5.0 MB.

No functional change.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Build cleanup — none of the above categories apply.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation — n/a, no documented behaviour changes
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules — n/a

Verification

Both build paths were built locally on macOS arm64:

GDExtension (./scripts/build-extension.sh) — builds and links clean.

libSSGodot.macos.editor          4,538,936 bytes
undefined flatbuffers symbols    0  (fully resolved)
flatc-side symbols in binary     0  (Parser / GenerateText / Registry)

Custom module (./scripts/build.sh) — godot.macos.editor.arm64 links successfully (2m32s). flatbuffers:: symbols in the binary: 0, as expected for header-only, fully-inlined use.

The duplicate symbol link warnings (Rust runtime symbols shared between libssruntime.a and libssconverter.a) are pre-existing and identical before and after this change.