#1 (parse_shebang args leading whitespace): `splitn(2, [' ', '\t'])`
discarded the whitespace separator, so an upstream-format `#!/bin/sh -e`
parsed to `args="-e"` and the rendered shim text emitted one space
between prog and the first flag where upstream emits two. Replace
`splitn` with a manual `find([' ', '\t'])` + `split_at` that keeps the
separator in the args slice, matching upstream's regex group `(.*)$`.
Update `parses_direct_shebang` and `parses_env_dash_s_shebang` to
assert the leading-space form so the regression direction is pinned.

#2 (idempotent skip checked only `.sh` but skipped writes for all
three): if a previous install wrote `.sh` correctly but `.cmd`/`.ps1`
were missing — older pacquet, partial-write crash — `already_correct`
short-circuited and the upgrade path never repaired the siblings.
Gate `already_correct` on the `.sh` marker AND the existence of both
`.cmd` and `.ps1`. Add `link_bins_rewrites_when_only_sh_flavor_exists`
which deletes the two siblings and verifies the second `link_bins`
pass re-creates them. Verified by re-breaking the gate temporarily
and confirming the new test fails, then reverting.

Findings 3, 4, 5, 9 are pending review acceptance; they'll land in a
follow-up commit.

…ecycle scripts (#397) (#418)

## Summary

Closes three of the critical items in #397 by porting the corresponding
behaviors from `@pnpm/npm-lifecycled2d8e790` and `pnpm/pnpm@b4f8f47ac2`:

- **#1 — Lifecycle env vars.** New `make_env` module ports `makeEnv` and
  the surrounding env block in `lifecycle()` ([npm-lifecycle/index.js:73-104](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/index.js#L73-L104), [:354-414](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/index.js#L354-L414)) plus the pnpm wrapper's `extraEnv` additions ([runLifecycleHook.ts:119-124](https://github.com/pnpm/pnpm/blob/b4f8f47ac2/exec/lifecycle/src/runLifecycleHook.ts#L119-L124)). Lifecycle scripts now see `npm_lifecycle_event`, `npm_lifecycle_script`, `npm_node_execpath`/`NODE`, `npm_package_json`, `npm_execpath`, `npm_package_*` (`name`, `version`, and recursive `config`/`engines`/`bin`), `npm_config_node_gyp`, `npm_config_user_agent`, `INIT_CWD`, `PNPM_SCRIPT_SRC_DIR`, and `TMPDIR` (when `unsafe_perm` is false). The spawn now strips inherited env (`env_clear()`) so leftover `npm_*` keys from a wrapping invocation cannot leak through.
- **#2 — PATH ancestor walk.** New `extend_path` module ports `extendPath` ([npm-lifecycle/lib/extendPath.js:5-27](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/lib/extendPath.js#L5-L27)) plus the tri-state `scriptsPrependNodePath` gating ([:29-61](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/lib/extendPath.js#L29-L61)). For a dep at `<root>/node_modules/.pnpm/<slot>/node_modules/<pkg>`, `PATH` now contains the dep's own `.bin`, the `.pnpm` slot's `.bin`, the project root's `.bin`, the bundled `node-gyp-bin` (when supplied), `extra_bin_paths`, and finally the inherited PATH.
- **#4 — Shell selection.** New `shell` module ports the [shell-selection block](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/index.js#L241-L252) and the [pnpm-side `.bat`/`.cmd` guard](https://github.com/pnpm/pnpm/blob/b4f8f47ac2/exec/lifecycle/src/runLifecycleHook.ts#L63-L71). `cmd /d /s /c` on Windows, custom `scriptShell` on either platform, otherwise `sh -c`. The Windows-batch-file `scriptShell` case surfaces as `ERR_PNPM_INVALID_SCRIPT_SHELL_WINDOWS` (matching upstream's error code).

`RunPostinstallHooks` grows seven new fields to surface these knobs; `BuildModules` passes safe defaults (`None` / `true` / `Never`) for all of them — full config plumbing for `user-agent`, `unsafe-perm`, `scripts-prepend-node-path`, `node-gyp` bundling, and `script-shell` are tracked as separate items in #397 (#14, #15) or follow-ups.

### Explicit non-goals in this PR

Three caveats called out in #397 that are deliberately deferred:

- `windowsVerbatimArguments` (Rust equivalent: `CommandExt::raw_arg`) is signalled by `SelectedShell.windows_verbatim_args` but not yet applied to the spawned `Command`.
- `@yarnpkg/shell` / `shellEmulator: true` has no clean Rust port; pacquet ignores the flag for now.
- `unsafe_perm` uid/gid drop (#14) — `BuildModules` passes `true`, which keeps current behavior (no TMPDIR creation, no privilege drop).

### Test parity

Per the project guide ("port the relevant pnpm tests too whenever they translate"), this branch ports:

- `test('makeEnv')` from [npm-lifecycle/test/index.js:97-124](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/test/index.js#L97-L124).
- The `extendPath` ordering test from [npm-lifecycle/test/extendPath.test.js:5-8](https://github.com/pnpm/npm-lifecycle/blob/d2d8e790/test/extendPath.test.js#L5-L8).
- `runLifecycleHook() does not set npm_config env vars` from [pnpm/exec/lifecycle/test/index.ts:65-77](https://github.com/pnpm/pnpm/blob/b4f8f47ac2/exec/lifecycle/test/index.ts#L65-L77), adapted to a file-dump model so we don't need the IPC fixture.
- `onlyOnWindows('pnpm shows error if script-shell is .cmd')` from [pnpm/exec/commands/test/index.ts:509-542](https://github.com/pnpm/pnpm/blob/b4f8f47ac2/exec/commands/test/index.ts#L509-L542) and the custom-shell case from [:478-508](https://github.com/pnpm/pnpm/blob/b4f8f47ac2/exec/commands/test/index.ts#L478-L508).

Plus fills the gaps where upstream coverage is thin: tri-state `scriptsPrependNodePath`, two-level pnpm virtual-store PATH walks, `extra_bin_paths` ordering, TMPDIR gating on `unsafe_perm`, and `extra_env` precedence vs `npm_lifecycle_script`.