fix(pineToJS): allow the _ discard identifier to be declared multiple times

[alexgrover]

Problem

Pine Script treats _ as a write-only discard identifier. Per the TradingView docs (Variable declarations → Using an underscore (_) as an identifier):

You can assign any number of values to a _ identifier anywhere in the script, even if the current scope already has such an assignment.

PineTS only de-duplicated _ within a single tuple, so any script that declared _ more than once in the same scope failed at the JS stage:

//@version=5
indicator("t")
[_, s1, _] = ta.macd(close, 12, 26, 9)
[_, s2, _] = ta.macd(close, 5, 10, 3)   // SyntaxError: Identifier '_' has already been declared
_ = ta.sma(close, 10)                    // same

Reproduced failures (before the fix):

• two tuple declarations re-using _ in the global scope
• the same inside a user function body
• plain _ = expr repeated
• tuple _ followed by plain _
• _ in an if branch followed by another _ declaration after the if

Fix

src/transpiler/pineToJS/codegen.ts: every _ declaration target — plain declarations, tuple declarations, and for [..] in headers — is renamed to a fresh _$N placeholder via a shared renameDiscardTargets / freshDiscardName helper. $ is not a legal Pine identifier character, so the placeholders can never collide with user variables (the same _$N scheme the codegen already uses for reserved-word renames).

Generated JS for the script above:

let [_$0, s1, _$1] = ta.macd(close, 12, 26, 9);
let [_$2, s2, _$3] = ta.macd(close, 5, 10, 3);
let _$4 = ta.sma(close, 10);

The two previous copy-pasted intra-tuple dedup loops are replaced by the helper.

_ stays write-only

TradingView also says "A value assigned to such a variable cannot be accessed" — reading _ is a compile error there (Undeclared identifier "_"). Because every declaration target is renamed, a read of _ in the generated JS is a genuinely undeclared identifier and fails with ReferenceError: _ is not defined — the same way PineTS surfaces every other undeclared identifier. (Before this PR the first _ in a tuple kept its name, so plot(_) silently returned the discarded value.) Tests pin this behavior.

Tests

New tests/transpiler/underscore-discard-identifier.test.ts (11 tests):

• 7 positive cases covering all the scenarios above plus the TradingView docs ta.bb example and a for-in loop body. Expected values come from the same computations written with uniquely-named variables (already-supported Pine), so the tests assert correctness of the resulting series, not just "no throw". All failed before the fix with Identifier '_' has already been declared.
• 3 negative cases asserting that reading _ (plot(_), _ + 1, inside a function body) throws.
• 1 codegen check that no bare _ declaration survives and each placeholder is unique.

Full suite: npm test -- --run → 175 files / 1863 tests passed, 0 failures.

Notes

• var [_, a, _] = ... is still rejected by the parser (Expected identifier after var). This matches TradingView, which does not allow var/varip on tuple declarations.
• _ := 5 (reassigning _) is not caught. This is pre-existing general behavior — any := to an undeclared name (e.g. foo := 5) currently becomes a sloppy-mode global rather than an error — and is not specific to _, so it is left out of this PR.
• No runtime-transpiler (stage 2) changes.

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