[Docs]: Fix misleading install instructions and unsubstantiated ARM64 performance claims

[membphis]

Background

Two accuracy problems in README.md:

1. luarocks install lua-qjson instruction is broken. README line 21 lists luarocks install lua-qjson as the install method, but luarocks.org/modules/api7/lua-qjson returns 404 — the package was never published. The v0.1.0 tag exists locally but users following README will hit an install failure. Local rockspec installation (luarocks make or luarocks install ./rockspec/...) works, but docs don't mention it.

2. ARM64 NEON support claimed without supporting data. The Status section (line 7) mentions "ARM64 NEON/PMULL structural scanner (runtime-dispatched)" alongside AVX2/PCLMUL, suggesting equivalent maturity. But all benchmark data (README and docs/benchmarks.md) is x86_64 only (AMD EPYC Rome, Zen 2). NEON support exists in code (src/scan/neon.rs, tests/scanner_crosscheck.rs validates correctness) but has zero published performance data. Users evaluating qjson for ARM deployment have no performance expectations to go on.

Goal

README install instructions must work; the ARM64 NEON claim must either be backed by benchmark data or explicitly qualified with its current limitations.

Non-goals

• Publishing the package to luarocks.org (separate Issue)
• Adding CI benchmark gates for ARM (separate Issue)

Acceptance Criteria

• Following the README install instructions, users can successfully install qjson (no longer depends on the missing luarocks.org package)
• The ARM64 NEON documentation claim matches its actual deliverable: either ARM benchmark data is provided, or the Status section explicitly notes "correctness-tested, no published performance benchmarks"

Task Checklist

1. Fix install instructions

• 1.1 Verify local rockspec install path works: confirm luarocks make or luarocks install ./rockspec/lua-qjson-0.1.0-1.rockspec succeeds
• 1.2 Update README ## Installing section, replacing luarocks install lua-qjson with the local install approach
• 1.3 If publication to luarocks.org is planned, add a note stating current status ("not yet published to luarocks.org")

2. Clarify ARM64 status

• 2.1 Decide approach: add ARM benchmark data or add a one-line qualification
• 2.2 Update README Status section to align the ARM64 claim with actual deliverable status
• 2.3 If "qualification only" approach chosen, also update docs/benchmarks.md Environment table (explicitly note x86_64 only)

3. Validation

• 3.1 Execute the README install instructions locally and confirm success
• 3.2 Read README end-to-end, confirm no remaining inconsistent claims

Notes / Decisions

• Code-level ARM64 NEON support exists and has correctness tests (tests/scanner_crosscheck.rs — proptest validates AVX2/NEON/scalar output identity). No code changes needed.
• Prior Issue [Docs]: Fix stale references, incomplete content, and benchmark number mismatches #55 fixed similar documentation consistency issues (CLAUDE.md stale references + benchmark number alignment). This Issue follows the same strict standard.

[coderabbitai]

🔗 Related PRs

#43 - test: add cjson and simdjson fixture coverage [merged]
#45 - feat: release v0.1.0 [merged]
#51 - perf: skip redundant validation in eager decode, inline depth check [merged]
#54 - perf: Lua encode + cache + scanner + validation stack optimizations [merged]
#56 - docs: fix stale references, incomplete content, and benchmark number mismatches [closed]

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