Add dynamic image to sixel api#2
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The image<> template requires compile-time W and H, which is awkward
for callers that produce arbitrary-resolution frames at runtime
(terminal previews, decoded video). Add two free functions in
namespace constixel that take RGBA bytes plus runtime width/height
and emit a sixel stream via a char-callback:
rgba_to_sixel(rgba, w, h, char_out)
Quantises to constixel's built-in 256-color palette
(format_8bit::quant). Use when input has more than 256 distinct
colors or when the caller has no preferred palette.
rgba_to_sixel(rgba, w, h, palette, char_out)
Uses a caller-supplied palette of up to 256 sRGB byte triples.
The built-in quantiser is bypassed; each pixel maps to its
squared-sRGB-distance nearest entry. Use when the caller already
knows the exact palette of the source data.
Implementation reuses the existing sixel header/raster/end helpers
in spirit but inlines a runtime-sized encoder loop with run-length
compression matching sixel_image<W,H,...>. No changes to the
compile-time-sized image<> path.
Replaces the freestanding rgba_to_sixel free functions with a proper sibling of image<F,W,H>: - runtime_palette: shared <=256-entry palette type (BGR-packed). - format_8bit_dyn: 8-bit indexed format with runtime W/H carried by parameter (vs format_8bit's compile-time W/H). Default palette delegates to format_8bit::quant; custom palette uses runtime_palette. - dynamic_image<Ops>: runtime-sized image, mirrors image<F,W,H> surface (blit_RGBA, sixel, sixel_to_cout under CONSTIXEL_ENABLE_COUT). Custom palette also wired into the existing static image<F,W,H> via new constructor overloads that take std::span<const std::array<u8,3>>, gated on the format exposing a runtime_palette typedef. blit_RGBA and sixel members dispatch through if-constexpr to *_with_palette helpers when has_custom_palette_ is set; the default code path is unchanged. format_8bit gains sixel_with_palette and blit_RGBA_with_palette static methods that delegate to format_8bit_dyn (defined out-of-line to break the circular header dependency). No duplication of the sixel encoder between compile-time and runtime image types.
Microsoft Terminal #17887: when P2 is 0 or omitted in the sixel introducer, an arbitrary number of cell-height rows past the image get filled with an arbitrary color (uninitialized memory). Setting P2=1 marks unset pixels as transparent and sidesteps the bug. For fully-painted images (every pixel covered, as dynamic_image produces) P2=0 vs P2=1 are visually identical — but only P2=1 renders correctly under WT >=1.22 sixel.
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