core: make the user's main.zig the executable root

[mattnite]

The firmware executable's root module is now the user's application source, not microzig's start.zig shim. This aligns microzig with the standard Zig build idiom: @import("root") from any module resolves to user code, pub const panic / pub const std_options live where Zig expects to find them, and tooling that assumes root-is-the-app now works naturally.

Core changes:

• microzig.zig reads microzig_options via @import("root") instead of the injected app module, and drops the pub const app alias.
• utilities.install_startup / core/src/start.zig are replaced by a single user-facing primitive, microzig.export_startup(), which the application opts into from a comptime block. It emits the CPU startup symbols and the microzig_main wrapper that the CPU _start calls once .data/.bss are initialized.
• Startup logic (HAL init / root init / error-returning main / panic wrapping) moves out of start.zig into microzig_main, with identical semantics.
• A new microzig.std_options() helper returns a std.Options with microzig's freestanding-safe defaults (the critical one being a no-op logFn that replaces stdlib's stderr-writing default, which fails to link on freestanding). The accompanying StdOptions struct documents which std.Options fields are exposed and which are intentionally excluded as hosted-only.
• The log-related fields (log_level, log_scope_levels, logFn) are removed from microzig.Options; they now live exclusively in std.Options via the helper.

Build system:

• add_firmware sets exe.root_module = app_mod directly. The separate start.zig root module and the "app"-import plumbing on core_mod / cpu_mod / chip_mod / hal_mod / board_mod are all gone.

Example boilerplate (now two re-exports and one comptime line):

pub const panic = microzig.panic;
pub const std_options = microzig.std_options(.{});

comptime {
    _ = microzig.export_startup();
}

All 131 example main.zig files have been migrated. Examples that previously declared pub const microzig_options with log-related fields have had those fields moved into microzig.std_options(...), with any remaining microzig-specific fields (interrupts, hal, cpu, etc.) kept in a trimmed pub const microzig_options.

The website getting-started snippet is updated to match.

This commit only performs the structural move. It is NOT expected to build against 0.15.x or current 0.16.x compilers as-is; the separate 0.16 migration work comes next.

[Grazfather]

hah whoa. Maybe we run the zig imports fix as a separate cleanup PR to cut down on the noise?

[mattnite]

@Grazfather nah, too easy to fix