stdlib: load from Lib/StLib sources, retire community stlib.kl

[pyrex41]

stdlib: load from Lib/StLib sources, retire the community stlib.kl overlay

Follow-up to the S41.2 refresh (#9). Makes Mark Tarver's S-lineage
Lib/StLib Shen sources the standard-library source of truth and
removes the pre-compiled community stlib.kl overlay the port had kept.

Why

The community ShenOSKernel-41.2 shipped the stdlib as a pre-compiled
stlib.kl. Installing those defuns bypassed the kernel's
load/define path, so stdlib functions ended up with the "unknown"
arity -1: bare (fn filter) and top-level (filter …) failed with
fn: filter is undefined. Tarver's refresh ships the stdlib as separate
sources loaded via install.shen; loading through the real load path
registers correct arities and package namespacing, fixing that.

What this PR does

• Vendors the S-lineage StLib sources under kernel/stlib/
  (byte-identical to mirror pyrex41/shen-upstream tag
  s41.2-pristine-20260711, Lib/StLib/; the 22 files install.shen
  loads). See kernel/stlib/PROVENANCE.md.
• Removes kernel/klambda/stlib.kl, its AOT module
  aot/kernel/stlib.rs, the stlib entries in aot/kernel/mod.rs, and
  the embedded stlib.kl in shenffi.
• Loads StLib at boot (boot::load_stlib, after the kernel is fully
  up) by running upstream's install.shen. Because install.shen uses
  relative (load …) and shen-rust resolves file paths against the
  process cwd, and because multiple interpreters can boot concurrently
  (cwd is process-global), it rewrites the load paths to absolute into a
  per-boot temp file and loads that — no cwd mutation, fully re-entrant.
  Runs under *hush* so the per-file chatter stays off the boot path.
• KERNEL_SLOW_DEFUNS is now empty (crates/klcompile): its only
  members were the four stlib.initialise-* defuns in stlib.kl.
• .cargo/config.toml sets RUST_MIN_STACK=256MB: the type-checked
  StLib load recurses past cargo's 2 MB test-thread stack (the binary
  already boots on a ≥64 MB thread). Makes cargo test / cargo run
  work without a hand-set env var.
• Regression test
  library.rs::stdlib_loaded_from_source_has_real_arities: (arity filter) >= 0, (fn filter) resolves, (filter …) filters, and the
  package-internal list.reduce is reachable namespaced.

Behavior notes (not regressions)

• Package-internal (non-exported) stdlib functions are namespaced:
  reduce lives in (package list …), reachable as list.reduce, not
  bare reduce (the community stlib.kl had no reduce at all). The
  test's map helper was renamed sq→my-square because sq is now a
  real StLib function and the kernel refuses to redefine a system fn.

• Because the load re-runs install.shen in full — including its closing
  (preclude-all-but []) — the StLib datatype recognisers (…#type) do
  not persist into the global namespace, so they can't shadow a user/test
  datatype (e.g. the print#type vs c-minus print clash other ports
  hit when baking). Immune by construction; kernel-tests stay 134/134.

Upstream StLib source issues

None. (An earlier concat* is not a legitimate function name was a
double-load artifact — loading install.shen a second time over an
already-installed stdlib, where concat* is by then a system function.
A clean single boot loads it without error.)

Test status

Gate	Result
0 shengen-codegen	PASS
1 fmt + clippy	PASS
2 build (workspace)	PASS
3 test (port-authored)	PASS
4 shen-check	PASS
5 tcb-audit	PASS
6 kernel-aot-audit	PASS
7 kernel-tests (release)	134/134
8 kernel-tests (debug)	134/134
9 kernel-tests (debug-gc)	134/134
10 cedar-equiv	PASS

Follow-up

shen_boot_embedded (shenffi, no-filesystem iOS path) does not yet
bundle the StLib sources, so that specific path brings up the kernel
without the stdlib. Embedding the sources for the no-fs boot is future
work (flagged in shenffi/src/lib.rs and kernel/stlib/PROVENANCE.md).

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