fix(sample_ntt,sha3): FIPS-203 SHAKE-128 squeeze + self-checking sha3 TBs

sample_ntt was non-conformant: both RTL and the test reference re-ran
keccak_p after every 3-byte squeeze instead of consuming the full
1344-bit SHAKE-128 rate. Only coeff[0] matched a standard sampler, so
the generated A matrix would not interoperate with any compliant ML-KEM.

- sample_ntt_sync{,_shared}.v: walk all 56 groups of the rate block via
  grp_ptr_r; re-permute only when the block is exhausted. Verified
  256/256 against ml-kem-r Rust sample_ntt on two seeds, and 1536/1536
  in the Verilator framework (runtime ~128x faster per poly).
- gen_vectors.py: use a self-contained hashlib.shake_128 oracle.

sha3 testbench fixes (all now self-check hash_o against verified vectors,
cross-checked with hashlib and ml-kem-r mlkem_G):
- tb_sha3_xsim_simple.v: test G/H/J modes, not just G.
- tb_keccak_core_xsim.v: correct the wrong EXPECTED_STATE constant
  (RTL was correct; lane0 = 0xf1258f7940e1dde7 per FIPS 202).
- tb_sha3_xsim.v: read expected file and self-check per vector; add
  vectors/g_basic_{input,expected}.hex (3 G / 2 H / 2 J).

Remove stale sha3_chain test (its RTL was deleted in 1cace51) and its
README references. Extend .gitignore for XSIM artifacts and result dumps.

sync_rtl/sha3/TB/vectors/g_basic_input.hex

@@ -0,0 +1,7 @@
+00000000000000000000000000000000000000000000000000000000000000007DBC2AC0D13D719B37B4E2D4691951FF890A97854EF5D3A8957EF67A54978E26C9
+0000000000000000000000000000000000000000000000000000000000000000C247885BF4E257CF39645D34C593047B7570D6ABBA300599D96171A950BB5027D5
+0000000000000000000000000000000000000000000000000000000000000000B038CB3C2F0F7398474A3D68C4C90B777F42FA4C12B1FC8F70E1DAADF20755473C
+010000000000000000000000000000000000000000000000000000000000000000F9D94DE0F1EF4D2CB73DCA0F437D2334320494E5D0F728D73F5275E342572FF9
+0100000000000000000000000000000000000000000000000000000000000000008A1733CED3BC820A5D410AD00B0E4ACB0C260A8EBCD1B70FECF85B59256609CB
+02CC2F7B1D60813526FB623E09F10D22635605046A66888277667DACCA02B420BBE795DC9A6DA171DA1DE082111568D55A8A27FE9B009275761098B44B484B3684
+02E94BA9958BE80EDB47A9C83C07DBE09EA32CF4B1A59AD014938117DA74B87E9840E880E1A6F0C9E69C5E78D61A357D30ECC92FE5890DF046D93D94E87EA4ECB0