feat: native Windows HNSW support (replace renameio with natefinch/atomic)

[nvandessel]

Problem

coder/hnsw depends on google/renameio which does not compile on Windows. Currently, Windows builds fall back to BruteForceIndex (O(n) search, no persistence) via build tags (hnsw_windows.go).

Root Cause

The renameio dependency is used in exactly one function: SavedGraph.Save() in encode.go for atomic file writes. google/renameio will never support Windows — this is a deliberate design decision.

Proposed Solution

Fork coder/hnsw and replace renameio with natefinch/atomic, which supports Windows via MoveFileExW. The change is ~20-30 lines in a single file.

The coder/hnsw maintainer has described exactly what he wants in a previous (closed) PR:

1. Replace renameio.TempFile + CloseAtomicallyReplace with atomic.WriteFile
2. Use io.Pipe + bufio buffering on both sides to avoid doubling peak memory
3. g.Export(wr) writes into pipe writer, atomic.WriteFile(path, pipeReader) reads from pipe reader

Implementation Plan

1. Fork coder/hnsw → nvandessel/hnsw
2. Apply the natefinch/atomic + io.Pipe fix
3. Add replace directive in go.mod
4. Delete hnsw_windows.go and remove //go:build !windows tags from hnsw.go / hnsw_test.go
5. Submit the same fix upstream as a PR to coder/hnsw (ref: issue #9)
6. Once merged upstream, remove the replace directive

Context

• coder/hnsw#9 — Windows Compatibility (closed without fix)
• coder/hnsw#12 — Previous PR (closed due to tone, not approach)
• google/renameio#1 — Windows will never be supported
• Current workaround: internal/vectorindex/hnsw_windows.go (BruteForceIndex fallback)

Alternatives Considered

Option	Verdict
google/renameio/v2	Does not support Windows either
fogfish/hnsw	Different API, low adoption (18 stars), no built-in persistence
Bithack/go-hnsw	Abandoned since 2017
Keep brute-force fallback	Acceptable short-term but doesn't scale