[bug] t27o: str_chain never opens with a text field, so let t = n.value.trim(); t.starts_with(..) is refused while the inline chain converts #10834

What is wrong

str_chain (specs/tools/t27o/rust_convert.t27) is meant to tell that let t = node.value.trim(); binds a text when value is a text field of node's struct (its comment: "the chain may open with a text field of the head name's own type"; recv_class does the same for an inline chain). Its field branch tests the wrong thing:

if (f < src.len and is_alpha(src[f]) and fe <= b and tok_is(src, skip_ws(src, fe), "(")) {
    base = field_of_class(src, st, a, e, f, fe) == C_STR;
    c = skip_ws(src, fe);
}

It asks for a ( right after the field name, i.e. node.value( -- a method call, not a field -- and, when that holds, the loop that follows needs a . at c and returns false on the (. For node.value.trim() the token after value is ., so the branch is never taken, base stays false and str_chain answers false. recv_class's same step reads !tok_is(src, skip_ws(src, f0e), "(").

Repro (t27o of the branch of #10817)

pub struct Node { pub value: &'static str }
pub fn f(n: &Node) -> bool { let t = n.value.trim(); t.starts_with("a") }
pub fn g(n: &Node) -> bool { n.value.trim().starts_with("a") }

tri t27o converts g and refuses f: method starts_with``. The only difference is that f goes through the `let`, i.e. through name_class -> str_chain.

Expected

f converts like g: t is a text, so t.starts_with("a") takes the text entry of the std table.

Found by

Writing the direct tests of str_chain (lane C1 of #10522, issue #10825). The test that shows it is known_defect_str_chain_does_not_open_with_a_text_field in rust_convert.t27: it pins the current answer and flips when this is fixed.

User Scenarios

• Given a Rust function f with let t = n.value.trim(); t.starts_with("a") where n.value is a text field, when t27o converts it, then the conversion succeeds because str_chain recognizes t as text via the field access.
• Given a Rust function g with n.value.trim().starts_with("a") (inline chain), when t27o converts it, then the conversion succeeds as it does today.

Requirements

• FR-001: str_chain in specs/tools/t27o/rust_convert.t27 MUST treat a field access (.) after the field name as a valid continuation for a text field, not require a method call (().
• FR-002: The field branch condition MUST match recv_class's logic: allow . after the field and reject ( immediately after the field.
• FR-003: The existing test known_defect_str_chain_does_not_open_with_a_text_field MUST flip from pinning the current (wrong) answer to expecting the fixed answer.

Success Criteria

• tri t27o on the repro snippet converts both f and g without error (exit 0).
• The test known_defect_str_chain_does_not_open_with_a_text_field in specs/tools/t27o/rust_convert.t27 passes (1 test).
• t27c specs/tools/t27o/rust_convert.t27 compiles the spec without error (exit 0).

Boundary

• specs/tools/t27o/rust_convert.t27