update: host function renames

[mvadari]

High Level Overview of Change

Context of Change

Type of Change

• New XLS Draft
• XLS Update (changes to an existing XLS)
• XLS Status Change (e.g., Draft → Final, Draft → Stagnant)
• Process/Meta (changes to CONTRIBUTING.md, XLS-1, templates, etc.)
• Infrastructure (CI, workflows, scripts, website)
• Documentation (README updates, typo fixes)

[Copilot]

Pull request overview

Updates the XLS-0102 WASM VM specification to reflect a proposed host function renaming scheme (shorter function names and “keylet” → “index” terminology) across the documented host API surface.

Changes:

• Renames ledger header/amendment/fee host functions (e.g., get_ledger_sqn → ldgr_index, get_base_fee → base_fee).
• Renames field-access host functions to the new “_inner” / “entry*” naming scheme and updates the “Keylets” section to “Indexes”.
• Renames several utility/trace/float host functions (e.g., compute_sha512_half → sha512_half, float_compare → float_cmp, trace_opaque_float → trace_xfloat).

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