[HAL] Refactor HAL executable exports to function IDs (#24507)

Refactor the durable HAL runtime API from export ordinals to
executable-local function ids while leaving compiler dialect churn
minimal. Function ids are carried as 64-bit C and VM tokens so future
backends can use executable-local encodings; current dense-table
implementations validate and decode them as indexes.

Public rename map:

- iree_hal_executable_export_count -> iree_hal_executable_function_count
- iree_hal_executable_export_info -> iree_hal_executable_function_info
- iree_hal_executable_export_parameters ->
iree_hal_executable_function_parameters
- iree_hal_executable_lookup_export_by_name ->
iree_hal_executable_lookup_function_by_name
- iree_hal_executable_export_info_t ->
iree_hal_executable_function_info_t
- iree_hal_executable_export_parameter_t ->
iree_hal_executable_function_parameter_t
- iree_hal_executable_export_flags_t ->
iree_hal_executable_function_flags_t
- IREE_HAL_EXECUTABLE_EXPORT_FLAG_* ->
IREE_HAL_EXECUTABLE_FUNCTION_FLAG_*
- IREE_HAL_EXECUTABLE_EXPORT_PARAMETER_* ->
IREE_HAL_EXECUTABLE_FUNCTION_PARAMETER_*
- iree_hal_executable_vtable_t
export_count/export_info/export_parameters ->
function_count/function_info/function_parameters
- iree_hal_executable_vtable_t lookup_export_by_name ->
lookup_function_by_name
- HAL dispatch export_ordinal/entry_point arguments ->
function/function_id

Compiler HAL ops and attributes intentionally keep their existing export
names in this change. Runtime VM imports now use function_id, and
executable.lookup.function is exposed from the HAL and hal_loader
modules so callers can resolve stable function names to runtime ids
before dispatch.