refactor(tst-c): collapse doubled cbindgen feature guards to single

[aklofas]

Summary

Collapses the redundant doubled preprocessor guard
#if (defined(TST_HAS_X) && defined(TST_HAS_X)) that cbindgen emitted on
190 C function prototypes in bindings/c/include/tstrans.h down to a single
#if defined(TST_HAS_X).

Root cause: every extern-C fn in the rtp/rtsp/udp/tcp/rist surfaces carried a
per-function #[cfg(feature = "X")] on top of its already-#[cfg(feature="X")]-gated
module (declared in bindings/c/core/src/lib.rs). cbindgen ANDs the module-path cfg
with the item cfg, producing the doubled guard. The SRT and HLS surfaces never had
per-fn cfgs and already produced single guards — they were the correct template.

This resolves the cbindgen double-guard comments raised on PR #40 (deferred
follow-up of the chunk-#2 polish pass).

Change

• Remove the 190 redundant per-function #[cfg(feature = "X")] from the extern-C fns
  under bindings/c/core/src/{rtp,rtsp,udp,tcp,rist}/** (RTP 83, TCP 39, UDP 34,
  RIST 34). Pure deletions — every removed line exactly duplicated its module's
  feature; no compound cfg (all(...)/not(...)/cross-feature) was touched, and no
  #[cfg(test)] was removed.
• Regenerate tstrans.h; all 190 doubled guards collapse to single guards.
• Refresh the now-stale explanatory comments in
  scripts/check/c/header-conditional-sections.sh (they described the per-fn cfg as
  the gating mechanism; the module cfg is).

Gating is provably unchanged

The module-level gate in lib.rs remains the sole guard.

• nm on the cdylib: --no-default-features exports zero transport symbols;
  --features udp exports exactly the 34 udp symbols and 0 rtp/tcp/rist.
• All 8 feature modes compile (--no-default-features, each single
  --features {srt,rtp,udp,tcp,hls,rist}, --all-features).
• The committed header's tst_*( function-name set is byte-identical apart from the
  guard collapse (428 prototypes both sides); the top #define TST_HAS_X 1 block is
  unchanged (still SRT + RTP only).

Verification (local, all green)

fmt; clippy --workspace --all-targets --all-features -D warnings; cargo test --workspace --all-features (3054 passed) and --no-default-features (2674 passed);
doctests; nightly-doc; cargo public-api clean ×8; #[non_exhaustive] 265; full
bash-ratchet sweep incl. all C-header checks; fuzz check.

[Copilot]

Pull request overview

This PR removes redundant per-function Rust #[cfg(feature = "...")] attributes from C-ABI extern functions in feature-gated transport modules (rtp/rtsp/udp/tcp/rist), which eliminates cbindgen’s duplicated preprocessor guards in the generated C header and updates the associated header-check script commentary.

Changes:

• Delete redundant per-function #[cfg(feature = "...")] on extern "C" transport APIs, relying solely on the already feature-gated modules in bindings/c/core/src/lib.rs.
• Regenerate bindings/c/include/tstrans.h so doubled guards like #if (defined(TST_HAS_X) && defined(TST_HAS_X)) collapse to #if defined(TST_HAS_X).
• Refresh explanatory comments in scripts/check/c/header-conditional-sections.sh to reflect module-level gating as the mechanism.

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/check/c/header-conditional-sections.sh	Updates rationale/comments to match module-level feature gating.
bindings/c/include/tstrans.h	Regenerated header with single #if defined(TST_HAS_*) guards.
bindings/c/core/src/udp/sender.rs	Removes redundant per-function #[cfg(feature="udp")] on externs.
bindings/c/core/src/udp/receiver.rs	Removes redundant per-function #[cfg(feature="udp")] on externs.
bindings/c/core/src/udp/mux_sender.rs	Removes redundant per-function #[cfg(feature="udp")] on externs.
bindings/c/core/src/udp/demux_receiver.rs	Removes redundant per-function #[cfg(feature="udp")] on externs.
bindings/c/core/src/tcp/sender.rs	Removes redundant per-function #[cfg(feature="tcp")] on externs.
bindings/c/core/src/tcp/receiver.rs	Removes redundant per-function #[cfg(feature="tcp")] on externs.
bindings/c/core/src/tcp/mux_sender.rs	Removes redundant per-function #[cfg(feature="tcp")] on externs.
bindings/c/core/src/tcp/listener.rs	Removes redundant per-function #[cfg(feature="tcp")] on externs.
bindings/c/core/src/tcp/demux_receiver.rs	Removes redundant per-function #[cfg(feature="tcp")] on externs.
bindings/c/core/src/rtsp/server/stop.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtsp/server/start.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtsp/server/mount.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtsp/server/mount_getters.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtsp/server/builder.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtsp/client/session.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtp/sender.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtp/receiver.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtp/mux_sender.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rtp/demux_receiver.rs	Removes redundant per-function #[cfg(feature="rtp")] on externs.
bindings/c/core/src/rist/sender.rs	Removes redundant per-function #[cfg(feature="rist")] on externs.
bindings/c/core/src/rist/receiver.rs	Removes redundant per-function #[cfg(feature="rist")] on externs.
bindings/c/core/src/rist/mux_sender.rs	Removes redundant per-function #[cfg(feature="rist")] on externs.
bindings/c/core/src/rist/demux_receiver.rs	Removes redundant per-function #[cfg(feature="rist")] on externs.

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