rust encoder: `while let` unsupported, panics with misleading "let-guard outside if/while"

[ahmednfwela]

What happens

rust/encoder/src/control_flow.rs::encode_while (and encode_loop) hands its
condition straight to self.encode_expr(&e.cond) with no special case for
syn::Expr::Let:

pub(crate) fn encode_while(&mut self, e: &syn::ExprWhile) -> Expression {
    let condition = self.encode_expr(&e.cond);
    ...
}

encode_if (same file, ~line 149) DOES special-case syn::Expr::Let for if let.
while let Some(x) = iter.next() { .. } — an extremely common Rust idiom for
draining an iterator/receiver — falls through encode_expr's match arms all the
way to the catch-all in lib.rs, which panics with:

syn::Expr::Let(_) => "let-guard outside if/while",

The message is actively misleading (it fires inside a while), and more importantly
there is no support for while let in the Rust encoder at all — every
non-trivial file that drains an iterator or a channel with while let cannot be
encoded, loud but for the wrong stated reason.

Why the tests did not catch it

This was surfaced by a reviewer of PR #698 as a pre-existing gap in
encode_while/encode_expr's fallback, flagged and explicitly left out of scope
in that PR's body. No test in rust/encoder/tests/ exercises a while let
expression, so nothing pins either the missing feature or the wrong panic message.

Definition of done

• RED-first test(s) in rust/encoder/tests/ (loops/control-flow suite) that
  encode a while let Some(x) = <iter-expr> { .. } program and assert either a
  correct std.while-based encoding (mirroring encode_if's Expr::Let
  handling — Ball's while has no pattern-binding condition, so this likely
  needs an Option/outcome-shaped desugaring similar to how for loops desugar
  iteration) or, if genuinely out of scope for now, a loud refusal that correctly
  names while let rather than "let-guard outside if/while".
• rust/encoder/src/control_flow.rs::encode_while (and encode_loop if
  applicable) updated to match; .claude/rules/rust.md documents the resulting
  behavior.
• cargo test -p ball-lang-encoder, cargo fmt -p ball-lang-encoder -- --check,
  cargo clippy -p ball-lang-encoder --all-targets -- -D warnings all clean.

Refs

PR review: #698 (comment)
Evidence: rust/encoder/src/control_flow.rs:360-366 (encode_while), :149
(encode_if's existing Expr::Let handling), rust/encoder/src/lib.rs:2751
("let-guard outside if/while") — all origin/main @ b8ae4e6.