README: describe every constant LibCodeGen emits, not only function pointers

[thedavidmeister]

Closes #87

What was wrong

The README's opening said the library "hosts the constant caches for prebuilt
function-pointer tables". That is one of several things LibCodeGen emits, and
it is not what the repo's own committed example produces.

Confirmed against the source rather than taken from the issue:

• src/lib/LibCodeGen.sol declares 11 emitting functions (the *ConstantString
  set).
• Only 5 of the 11 concern function pointers (opcode, literal parser, operand
  handler, sub parser word parsers, integrity). The other 6 emit a
  deployed-bytecode hash, a described-by meta hash, and generic address,
  uint8, bytes32 and bytes constants.
• src/generated/CodeGennable.sol, the only committed example output and the
  artifact the README itself tells you to regenerate, holds a BYTECODE_HASH
  and two bytes constants and zero function pointers.

What changed

README.md opening paragraph, and nothing else:

-(pragma + foundry-clean formatting) that hosts the constant caches for prebuilt
-function-pointer tables — needed for runtime gas efficiency in the Rain
-interpreter.
+(pragma + foundry-clean formatting) hosting build-time constants that a contract
+imports and the compiler inlines: prebuilt function-pointer tables (the case
+that motivates it — runtime gas efficiency in the Rain interpreter), a
+deployed-bytecode hash, a described-by meta hash, and plain
+`address`/`uint8`/`bytes32`/`bytes` constants.

No test guards this, and it can regress silently

This change is not covered by any test and can regress silently. If someone
adds a twelfth emitter to LibCodeGen, or edits this paragraph back toward the
function-pointer-only description, or deletes it, the suite stays green and CI
stays green. Nothing in this repo compares the README against the emitter set.
The only thing standing between that drift and a reader is review of the diff.

This is not being rehomed. rainlanguage/rainix#317 collects the checks from
this sweep that a generic rainix-static lint could express; a hand-written map
from LibCodeGen's emitter names to English phrases in this repo's README is not
one of them, so #317 is not its home either. There is no follow-up issue and no
deferred coverage — the documentation fix lands unguarded, deliberately.

What this PR previously carried, and why it is gone

An earlier revision of this branch added test/src/lib/LibCodeGen.readme.t.sol
(252 lines) plus two fs_permissions read grants (README.md,
src/lib/LibCodeGen.sol) so it could vm.readFile both. That test has been
deleted and both grants reverted, so foundry.toml is now byte-identical to
main.

It was written because the brief driving this fleet of fixes said every fix
ships a test and verify-by-hand is never acceptable, with no exemption for
changes that have no behaviour. That instruction was wrong and has been
corrected. A README wording fix has no behaviour to test, and the test it
produced was hand-rolled static analysis written in Solidity:

• 170 of its 252 lines were eight internal string-scanning helpers, and 2 of its
  3 test functions asserted on those helpers rather than on anything in src/.
• It pinned seven README substrings, so it fought the next README edit rather
  than protecting the library.
• It restated LibCodeGen's declaration list as EXPECTED_EMITTER_COUNT = 11
  plus an order-sensitive name list, so reordering two functions — a change with
  no behaviour at all — would have turned the suite red.

QA

nix develop -c forge test:

Ran 18 test suites in 2.03s (18.23s CPU time): 142 tests passed, 0 failed, 0 skipped (142 total tests)

Before the strip (same merge commit, test file and grants still present):
Ran 19 test suites: 145 tests passed, 0 failed, 0 skipped (145 total tests) —
the drop is exactly the 3 functions removed, and every remaining test passes.

nix develop -c forge fmt --check: exit 0, no diff. pre-commit
(no-consumer-prettier, taplo, denofmt): Passed on the commit.

origin/main is merged in (merge commit, not a rebase); the branch predated
merged PRs #110 and #124.

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• README.md

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