Post-merge-review: Fix template-no-implicit-this: callee detection, block-param scoping, bare {{this}}

[johanrd]

What's broken on master

Four independent false positives:

1. SubExpression / ElementModifier / BlockStatement callees flagged. {{echo (my-helper)}}, <div {{my-modifier}}>, and {{#my-component}}…{{/my-component}} are all flagged as ambiguous this.* candidates. Upstream whitelists all callee positions via a nextPathIsCallee flag (no-implicit-this.js L104–151).
2. Block-param scope tracking only checks direct parent. {{#each @items as |item|}}{{item.name}}{{/each}} flags item.name because the scope walker stops at the direct parent. Fix walks the ancestor chain.
3. JS scope bindings flagged in GJS/GTS. import x from '…'; <template>{{x}}</template> or const cond = false; <template>{{if cond "y" "n"}}</template> produce false positives because the port doesn't check whether the identifier resolves to a JavaScript binding. The existing CONTROL_FLOW_HELPERS blanket-skip is a coarse workaround (it exempts all positional args of if/unless/each/etc., even truly-ambiguous ones).
4. Bare {{this}} flagged. Master's startsWith-check doesn't catch the exact string "this".

Fix

• Unified isCalleePosition that treats SubExpression / BlockStatement / ElementModifierStatement callees as always-valid, and MustacheStatement callees as valid only when the mustache has arguments.
• isLocalBlockParam walks up the ancestor chain collecting GlimmerBlockStatement.program.blockParams.
• isJsScopeVariable uses sourceCode.getScope() to detect JS bindings. Replaces the coarse CONTROL_FLOW_HELPERS workaround with precise detection: truly-ambiguous paths like {{if condition "y" "n"}} (no binding for condition) are now correctly flagged again. Pattern matches template-no-redundant-fn and template-no-restricted-invocations (isJsScopeVariable).
• Skip reporting when path === 'this'.

Test plan

• 84/84 tests pass on the branch
• 10 new valid tests covering SubExpression/ElementModifier/BlockStatement callees (HBS + GTS), 4 new valid tests for nested block params, 2 new valid tests for bare {{this}}, and 3 new valid tests for JS-scope bindings (const, import, const with {{#each}}). All fail on master.
• 1 new invalid test ({{if condition "yes" "no"}} with no JS binding) confirms control-flow args are correctly flagged when truly ambiguous. Fails on master (master silently allowed via CONTROL_FLOW_HELPERS).

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Co-written by Claude.