h264: promote Intra_4x4 luma prediction (9 modes) to public API

PR #12 added the 9 Intra_4x4 luma intra prediction modes as test-only
spec references in tests/.  This PR promotes them to public src/
symbols so consumers (the eventual marfrit-packages substitution-arc
patch 0014) can link against them.

  Moved: tests/h264_intra_pred_4x4_ref.c → src/h264_intra_pred_4x4.c
  Renamed: daedalus_h264_pred_4x4_<mode>_ref → daedalus_h264_pred_4x4_<mode>
           (9 functions: vertical/horizontal/dc/ddl/ddr/vr/hd/vl/hu)

The src/ implementation is byte-for-byte the same code as the
test-only ref; this PR is plain plumbing.  The test binary now
links against daedalus_core to pull in the public symbols (instead
of compiling the ref file directly), exercising the path that real
consumers will use.

Same promotion shape as PR #25 (chroma DC Hadamard).

Verified on hertz:

  $ ./build/test_intra_pred_4x4
    Vertical (mode 0)          PASS
    Horizontal (mode 1)        PASS
    DC (mode 2)                PASS
    DiagDownLeft (mode 3)      PASS
    DiagDownRight (mode 4)     PASS
    VerticalRight (mode 5)     PASS
    HorizontalDown (mode 6)    PASS
    VerticalLeft (mode 7)      PASS
    HorizontalUp (mode 8)      PASS
    VR asym (sanity)           PASS

  ALL 10 intra-4x4 mode references PASS

  $ nm -g build/libdaedalus_core.a | grep "T daedalus_h264_pred_4x4"
  (9 symbols exported)

Follow-ups (same promotion pattern, can land in parallel):
  - Intra_16x16 luma (4 modes, PR #13)
  - Intra_8x8 chroma (4 modes, PR #14)
  - Intra_8x8 luma (9 modes, PRs #21 + #22)

Once all 26 intra modes are in the public API, the marfrit-packages
substitution arc can route H264PredContext's pred function pointer
tables through daedalus alongside the IDCT / deblock / qpel / DC
Hadamard substitutions already in place.

include/daedalus.h

@@ -559,6 +559,33 @@ DECLARE_QPEL_AVG(avg_mc33)
  * ----------------------------------------------------------------- */
 void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4]);
 
+/* -------------------------------------------------------------------
+ * H.264 Intra_4x4 luma prediction (per H.264 §8.3.1.4).  9 modes.
+ *
+ * Pure CPU primitives — each is a small straightforward fill of a
+ * 4x4 output block from neighbour pixels in the same buffer.  No
+ * substrate-dispatch wrapper (the work is too small to amortise).
+ *
+ * FFmpeg-style interface: `dst` at row 0 col 0 of the 4x4 output.
+ * Reads top-left at dst[-stride-1], top at dst[-stride..-stride+7]
+ * (top-right for DDL/VL), and left at dst[r*stride - 1] for r=0..3.
+ * Caller must ensure all 13 neighbour bytes are valid (interior-MB
+ * assumption — H.264 availability fallback handled at caller).
+ *
+ * Bit-exact validated against tests/test_intra_pred_4x4.c (10-case
+ * spec-derived test suite including the asymmetric Vertical_Right
+ * 16-cell hand-derived case; see fourier PR #12).
+ * ----------------------------------------------------------------- */
+void daedalus_h264_pred_4x4_vertical  (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_horizontal(uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_dc        (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_ddl       (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_ddr       (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_vr        (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_hd        (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_vl        (uint8_t *dst, ptrdiff_t stride);
+void daedalus_h264_pred_4x4_hu        (uint8_t *dst, ptrdiff_t stride);
+
 /* -------------------------------------------------------------------
  * Recipe query — what does the API recommend for each kernel?
  * ----------------------------------------------------------------- */