fix(install): verified replaces, preserved MCP env, truthful self-update - #120

Reproduced with the exact README Windows installer: it downloaded and checksum-verified v0.9.18, printed [OK], but left the locked MCP exe at 0.9.16 and copied that stale binary over the newer Cargo copy.

Root cause

• install.ps1 Install-BinaryAtomic: Remove/Move failed silently on the locked exe; if (Test-Path) then mistook the surviving old binary for success and skipped the rename fallback.
• The PATH-copy step sourced from the (stale) primary destination, downgrading the other install.
• The final [OK] never compared installed vs requested version.
• MCP JSON upsert replaced the whole hashline entry (dropping env) and PS5 ConvertFrom-Json rejected case-distinct keys.

Fix

• Transactional replaces with terminating errors, unique staging/retire names, rollback, and SHA-256 verification; loud failure instead of false success.
• PATH copies sourced from the extracted release payload, verified, best-effort with accurate reporting.
• Final version gate before [OK].
• JSON merge preserves env/unknown fields and exact-case keys on PS 5.1 and 7; invalid configs skipped untouched.
• Self-update verifies installed bytes/version and rolls back on probe failure; unique Windows retirements; reports exact path, leaves other installs and running MCP sessions untouched.

Tests (isolated, no live config/PATH/network touched)

• scripts/test-installer.ps1: 19/19 pass (normal, locked delete-share, hard-lock loud failure, JSON fixtures incl. case-distinct keys, dotted amp key).
• cargo test -p hashline --test update_e2e --locked --offline: 10/10.
• cargo test -p hashline --lib --locked --offline: 337/337.
• cargo fmt --all --check, git diff --check: clean.
• install.ps1 parses clean on PS 5.1; bash -n install.sh clean.