Add SIMD indexByteTwo/lastIndexByteTwo doc

src/algo/SIMD.md

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+# SIMD byte search: `indexByteTwo` / `lastIndexByteTwo`
+
+## What these functions do
+
+`indexByteTwo(s []byte, b1, b2 byte) int` — returns the index of the
+**first** occurrence of `b1` or `b2` in `s`, or `-1`.
+
+`lastIndexByteTwo(s []byte, b1, b2 byte) int` — returns the index of the
+**last** occurrence of `b1` or `b2` in `s`, or `-1`.
+
+They are used by the fuzzy matching algorithm (`algo.go`) to skip ahead
+during case-insensitive search. Instead of calling `bytes.IndexByte` twice
+(once for lowercase, once for uppercase), a single SIMD pass finds both at
+once.
+
+## File layout
+
+| File                  | Purpose                                                           |
+| ------                | ---------                                                         |
+| `indexbyte2_arm64.go` | Go declarations (`//go:noescape`) for ARM64                       |
+| `indexbyte2_arm64.s`  | ARM64 NEON assembly (32-byte aligned blocks, syndrome extraction) |
+| `indexbyte2_amd64.go` | Go declarations + AVX2 runtime detection for AMD64                |
+| `indexbyte2_amd64.s`  | AMD64 AVX2/SSE2 assembly with CPUID dispatch                      |
+| `indexbyte2_other.go` | Pure Go fallback for all other architectures                      |
+| `indexbyte2_test.go`  | Unit tests, exhaustive tests, fuzz tests, and benchmarks          |
+
+## How the SIMD implementations work
+
+**ARM64 (NEON):**
+- Broadcasts both needle bytes into NEON registers (`VMOV`).
+- Processes 32-byte aligned chunks. For each chunk, compares all bytes
+  against both needles (`VCMEQ`), ORs the results (`VORR`), and builds a
+  64-bit syndrome with 2 bits per byte.
+- `indexByteTwo` uses `RBIT` + `CLZ` to find the lowest set bit (first match).
+- `lastIndexByteTwo` scans backward and uses `CLZ` on the raw syndrome to
+  find the highest set bit (last match).
+- Handles alignment and partial first/last blocks with bit masking.
+- Adapted from Go's `internal/bytealg/indexbyte_arm64.s`.
+
+**AMD64 (AVX2 with SSE2 fallback):**
+- At init time, `cpuHasAVX2()` checks CPUID + XGETBV for AVX2 and OS YMM
+  support. The result is cached in `_useAVX2`.
+- **AVX2 path** (inputs >= 32 bytes, when available):
+  - Broadcasts both needles via `VPBROADCASTB`.
+  - Processes 32-byte blocks: `VPCMPEQB` against both needles, `VPOR`, then
+    `VPMOVMSKB` to get a 32-bit mask.
+  - 5 instructions per loop iteration (vs 7 for SSE2) at 2x the throughput.
+  - `VZEROUPPER` before every return to avoid SSE/AVX transition penalties.
+- **SSE2 fallback** (inputs < 32 bytes, or CPUs without AVX2):
+  - Broadcasts via `PUNPCKLBW` + `PSHUFL`.
+  - Processes 16-byte blocks: `PCMPEQB`, `POR`, `PMOVMSKB`.
+  - Small inputs (<16 bytes) are handled with page-boundary-safe loads.
+- Both paths use `BSFL` (forward) / `BSRL` (reverse) for bit scanning.
+- Adapted from Go's `internal/bytealg/indexbyte_amd64.s`.
+
+**Fallback (other platforms):**
+- `indexByteTwo` uses two `bytes.IndexByte` calls with scope-limiting
+  (search `b1` first, then limit the `b2` search to `s[:i1]`).
+- `lastIndexByteTwo` uses a simple backward for loop.
+
+## Running tests
+
+```bash
+# Unit + exhaustive tests
+go test ./src/algo/ -run 'TestIndexByteTwo|TestLastIndexByteTwo' -v
+
+# Fuzz tests (run for 10 seconds each)
+go test ./src/algo/ -run '^$' -fuzz FuzzIndexByteTwo -fuzztime 10s
+go test ./src/algo/ -run '^$' -fuzz FuzzLastIndexByteTwo -fuzztime 10s
+
+# Cross-architecture: test amd64 on an arm64 Mac (via Rosetta)
+GOARCH=amd64 go test ./src/algo/ -run 'TestIndexByteTwo|TestLastIndexByteTwo' -v
+GOARCH=amd64 go test ./src/algo/ -run '^$' -fuzz FuzzIndexByteTwo -fuzztime 10s
+GOARCH=amd64 go test ./src/algo/ -run '^$' -fuzz FuzzLastIndexByteTwo -fuzztime 10s
+```
+
+## Running micro-benchmarks
+
+```bash
+# All indexByteTwo / lastIndexByteTwo benchmarks
+go test ./src/algo/ -bench 'IndexByteTwo' -benchmem
+
+# Specific size
+go test ./src/algo/ -bench 'IndexByteTwo_1000'
+```
+
+Each benchmark compares the SIMD `asm` implementation against reference
+implementations (`2xIndexByte` using `bytes.IndexByte`, and a simple `loop`).
+
+## Correctness verification
+
+The assembly is verified by three layers of testing:
+
+1. **Table-driven tests** — known inputs with expected outputs.
+2. **Exhaustive tests** — all lengths 0–256, every match position, no-match
+   cases, and both-bytes-present cases, compared against a simple loop
+   reference.
+3. **Fuzz tests** — randomized inputs via `testing.F`, compared against the
+   same loop reference.