loader: introduce component adapter and route backend calls

[saitoha]

Motivation

• Prepare for componentized loader refactoring by introducing a stable abstraction for loader backends so future factory/manager work can be layered without changing backend implementations.
• Preserve existing behavior while enabling an indirection point to inject runtime options to backends via a unified API.

Description

• Add a sixel_loader_component_t abstraction with vtable and thin wrappers (ref, unref, setopt, load, get_name) in src/loader-component.h / src/loader-component.c.
• Add a legacy adapter loader-component-legacy (src/loader-component-legacy.h / src/loader-component-legacy.c) that wraps existing sixel_loader_entry_t backends and maps setopt options to the legacy backend call signature.
• Change sixel_loader_load_file() to create a legacy component per candidate registry entry, apply loader options via setopt (loader_apply_component_options) and invoke decoding using component->load() instead of calling entry->backend(...) directly.
• Register the new sources and headers in both Meson and autotools build files (src/meson.build, src/Makefile.am, src/Makefile.in) so normal and amalgamated builds include the new files.

Testing

• Ran source /workspace/.local/env-autotools.sh && autoreconf -fi && ./configure && make -j4, and the build completed successfully (pass).
• Ran source /workspace/.local/env-autotools.sh && ./configure && make check, and the test suite completed with a single unrelated network test failure: io/network/0006_invalid_https_error_message (1 fail, other tests passed or skipped); this failure appears not to be related to the introduced loader component changes (⚠️).
• Ran Meson workflow meson setup builddir --wipe && meson compile -C builddir && meson test -C builddir, and the test run also produced the same single network test failure (most tests passed, 1 fail) (⚠️).
• Built amalgamated variants with Meson and autotools (meson setup builddir-amalgamated -Damalgamated_tools=true -Damalgamated_lib=true && meson compile -C builddir-amalgamated and ./configure --enable-amalgamated-lib --enable-amalgamated-tools && make -j4), and those builds completed successfully (pass).

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Codex Task

[coveralls]

coverage: 83.606% (+0.2%) from 83.377%
when pulling 8e40641 on feat/refactor-loader-mechanism-for-generalization
into 0f4b526 on develop.