Add plugin stencils, code coverage, compiling promises

[MatejKocourek]

Add plugin stencils, code coverage, moved RCP prologue to R runtime, specialize promises and optional support compiling promises.

[Copilot]

Pull request overview

This PR adds plugin stencil infrastructure for code instrumentation, implements code coverage tracking via srcref integration, moves RCP prologue handling to R runtime, and adds optional support for compiling promises to native code with specialization.

Changes:

• Introduced plugin stencil system allowing injection of custom code at specific bytecode positions
• Added code coverage infrastructure using srcref metadata and integration with the covr package
• Implemented specialized MAKEPROM operations with optional promise compilation to native code
• Refactored custom data handling (renamed PATCHED_VARIANTS to CUSTOM_DATA for general plugin use)

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
rcp/src/stencils/stencils.c	Added plugin coverage stencil, refactored MAKEPROM with specialization support and promise compilation
rcp/src/stencils/Makefile	Added compilation flags for MAKEPROM specialization and promise compilation
rcp/src/rcp_common.h	Renamed RELOC_RCP_PATCHED_VARIANTS to RELOC_RCP_CUSTOM for generalized plugin data
rcp/src/extractor/extract_stencils.cpp	Updated symbol extraction to use CUSTOM_DATA naming
rcp/src/compile.c	Implemented plugin stencil infrastructure, coverage tracking, peephole goto optimization, and promise compilation
rcp/src/Makevars	Added build flags for RCP_COMPILE_PROMISES and MAKEPROM_SPECIALIZE features
rcp/common.mk	Added configuration options for promise compilation and MAKEPROM specialization
external/rsh	Updated subproject commit reference

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[Copilot]

These printf debug statements should be replaced with DEBUG_PRINT macros for consistency with the rest of the codebase, allowing them to be conditionally compiled based on DEBUG_MODE.

Suggested change

	printf("Using specialized version of MAKEPROM_OP: BCODESXP\n");
	return &stencil_set[1];
	case EXTPTRSXP:
	if (RSH_IS_CLOSURE_BODY(code))
	{
	printf("Using specialized version of MAKEPROM_OP: EXTPTRSXP\n");
	return &stencil_set[2];
	}
	// Continue to the generic version if it's an external pointer but not ours
	default:
	printf("Using specialized version of MAKEPROM_OP: OTHER SEXPs\n");
	DEBUG_PRINT("Using specialized version of MAKEPROM_OP: BCODESXP\n");
	return &stencil_set[1];
	case EXTPTRSXP:
	if (RSH_IS_CLOSURE_BODY(code))
	{
	DEBUG_PRINT("Using specialized version of MAKEPROM_OP: EXTPTRSXP\n");
	return &stencil_set[2];
	}
	// Continue to the generic version if it's an external pointer but not ours
	default:
	DEBUG_PRINT("Using specialized version of MAKEPROM_OP: OTHER SEXPs\n");