[performance] cherry-pick: seg: 2-3x faster merge compression via Aho-Corasick matcher and cover-DP optimizations (#21625) - #21946

…-DP optimizations (#21625)

```
   words
     │
     ├──────────────────────────► [raw .idt file]   (every word, verbatim)
     │                                   │
     │  Phase 1          Phase 2         │  re-read in Phase 3
     └► mine patterns ─► reduce to ──► dictionary
                         top-K            │
                                  Phase 3 ▼
                          cover each word: patterns + leftover gaps
                                          │
                                  Phase 4 ▼
                          Huffman-code the patterns & positions
                                          │
                                  Phase 5 ▼
                          bit-pack everything ─────► [.seg file]
```

Merging storage-domain snapshot files is single-threaded and
compression-bound. This PR replaces the per-word pattern matcher and
optimizes the cover DP; output is byte-identical (or marginally smaller,
see notes).

Benchmarked on real storage files through `DomainRoTx.mergeFiles` (incl.
`.bt`/`.kvei` build), `Workers=1` as at chain-tip, merged content
verified against an independent re-merge:

| workload | main | this PR |
|---|---|---|
| bloatnet 16-step merge (766M in, 13.8M keys) | 382.6s | **135.0s
(2.8x)** |
| mainnet 6-file merge (1.61G in, 35.3M keys) | 1204.2s | **612.9s
(2.0x)** |

## Where the speedup comes from

CPU profile of a representative single-worker bloatnet 16-step merge,
attributing the cover phase by sub-component:

| phase | main | this PR | |
|---|---|---|---|
| cover phase (matcher + DP) | 241.2s | 14.6s | **16.5x** |
|   — matcher (`FindLongestMatches`) | 186.2s | 2.7s | ~68x |
|   — cover DP (excl. matcher) | 55.0s | 11.9s | ~4.6x |
| SAIS dictionary extraction | 89.2s | 87.5s | untouched |
| `.bt` + I/O + gc | ~19s | ~17s | untouched |
| **total** | **349.5s** | **119.5s** | **~2.9x** |

The matcher carries ~80% of the win, the cover-DP rewrite ~19%.
End-to-end speedup is Amdahl-capped near 3x: SAIS dictionary extraction
is untouched and is now ~79% of remaining merge time — parallelizing it
(size-neutral) is the next lever (see Notes).

The matcher's responsibility is - given the 64K limit candidate pattern
dictionary, run it through each word and find which pattern matches the
word. The cover DP finds optimal subset of these matches which provides
best compression.

## Changes

- `seg/patricia`: Aho–Corasick matcher replaces per-word suffix-array
matching (SAIS+LCP+bit-level patricia walk); same maximal-match set in
one O(len) scan, resuming from the shared prefix with the previous word
(merge keys are sorted)
- `seg`: `coverWordByPatterns` DP — single-match fast path; flat-slice
deque replacing `Ring`; upper-bound skip; monotone-bound early exit with
binary-search truncation; virtual initial window cells; position-code
maps → arrays

## Verification

- bloatnet output is byte-identical to main; mainnet output is 0.01%
smaller — mainnet's dictionary contains prefix-nested patterns that the
old matchers lose to a pre-existing patricia bug (#21626) and the AC
matcher finds; content verified equal in all runs
- `FuzzLongestMatch` extended with a brute-force oracle for the AC
matcher (3.5M+ execs clean)
- `make lint` clean; `db/seg`, `db/seg/patricia`, `db/state` tests pass

## Notes

- Remaining merge cost is dominated by dictionary learning (SAIS
extraction), untouched here; sampling it regresses mainnet sizes
(#21639, closed) — parallelizing extraction is the size-neutral
follow-up
- The earlier parallel existence-filter scan commit was dropped after
review: a second concurrent cursor can fight #21482's sequential-view
readahead on cold files
- Aho-Corasick can be parallelized by supplying ranges of words to each
compression worker, rather than feed consecutive words to different
workers -- when there is common prefix, AC can "resume" from previous
word with common prefix, thereby doing less work. Supplying ranges of
words exploit this. It'll be done in separate PR.

---------

Co-authored-by: Sudeep Kumar <sudeep.kumar@erigon.tech>
Co-authored-by: Alex Sharov <AskAlexSharov@gmail.com>

…rformance

# Conflicts:
#	db/seg/patricia/patricia_fuzz_test.go