fix(transpiler): + / - on display constants are set operations

[alexgrover]

Problem

Pine's display.* constants are a set type — + is the union of two displays and - removes one display's surfaces from another (display.all - display.price_scale is the reference manual's own example). PineTS keeps them as member-name strings, so:

• display.all - display.price_scale evaluated to NaN (hosts then fell back to their default — typically showing the plot everywhere);
• display.all + display.none produced the raw concatenation 'allnone', in source order, with duplicates (display.pane + display.pane → 'panepane').

Fix

• transformDisplayArithmetic — a post-process pass beside transformEqualityChecks — rewrites a + / - BinaryExpression with a display.<member> operand (a literal member, an expression this pass already rewrote, or a variable combined with a member) to display.__union(a, b) / display.__minus(a, b), built through ASTFactory. Numeric arithmetic elsewhere is untouched (pinned by test).
• displayUnion / displayMinus (Types.ts) compute the surface set and report it as the canonical concatenation of member names in the order pane, data_window, status_line, price_scale — 'all' / 'none' for the full / empty set. This is the shape + on the string enum already produced, so the display.* enum stays string-valued and hosts parsing member names keep working.

expression	result
display.all - display.none	all
display.all + display.none / display.none + display.all	all
display.none - display.all	none
display.all - display.price_scale	panedata_windowstatus_line
display.pane + display.data_window (either order)	panedata_window
display.all - display.pane - display.price_scale	data_windowstatus_line
display.pane + display.pane	pane

Tests

tests/namespaces/plot/display-arithmetic.test.ts — written failing first (NaN / 'allnone' / 'data_windowpane'), now green: the table above, chained and variable forms, hline / plotshape / input arguments, defaults untouched, and a transpiled-code check that only display expressions are routed. Full suite: 175 files / 1873 tests passing (3 pre-existing skips).

Docs: docs/api-coverage/types.md (display combination semantics), CHANGELOG.md under Unreleased.

Downstream

The Vela Pine engine (LuxAlgo/Vela-pinets#36) already parses this output; once this ships it can bump its pinets range so - becomes a guarantee.

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Note

Low Risk
Scoped transpiler rewrite and display helpers; fixes incorrect plot visibility strings without touching auth, data, or strategy logic.

Overview
Fixes Pine parity for combining display.* values: + and - now behave as set union and set difference instead of JavaScript string concatenation or NaN.

A new transpiler post-pass, transformDisplayArithmetic, rewrites + / - when either side is a display.<member> (including chained expressions and variables) into display.__union / display.__minus. Runtime helpers in Types.ts parse the canonical string form, compute surfaces in order pane → data_window → status_line → price_scale, and emit 'all' / 'none' or the joined member names hosts already understand. Plain literals and unrelated numeric math are unchanged.

Tests cover plot, hline, plotshape, and input display args; docs note set semantics in types.md.

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