Add rect_overlap and sprite_overlap collision detection helpers

[jonathanpeppers]

Add two new NESLib collision detection APIs as described in #421:

• rect_overlap(x1, y1, w1, h1, x2, y2, w2, h2) — Full AABB rectangle overlap test. Returns true if two axis-aligned rectangles overlap. Uses unsigned byte arithmetic, suitable for NES screen coordinates.

• sprite_overlap(x1, y1, x2, y2, threshold) — Distance-based collision for same-size sprites. Returns true if the unsigned absolute difference on both axes is less than the threshold.

Both are implemented as 6502 subroutines that read arguments directly from the cc65 parameter stack via indirect indexed addressing, avoiding expensive per-arg popa calls. They depend on addysp for stack cleanup.

Changes

• src/neslib/NESLib.cs — Added method stubs with XML docs
• src/dotnes.tasks/ObjectModel/BuiltInSubroutines.cs — 6502 assembly implementations
• src/dotnes.tasks/ObjectModel/Program6502.cs — Registered in ForEachOptionalBuiltIn with addysp dependency
• samples/climber/Program.cs — Replaced manual byte-distance collision with sprite_overlap
• samples/pong/Program.cs — Replaced nested paddle collision checks with rect_overlap
• Updated test DLLs and verified snapshots

Before/After (climber)

// Before: ~7 lines of manual collision
byte dx = (byte)(actor_x[0] - actor_x[ci]);
if (dx >= 248) dx = (byte)(0 - dx);
byte dyl = (byte)(actor_yy_lo[0] - actor_yy_lo[ci]);
if (dyl >= 248) dyl = (byte)(0 - dyl);
if (dx < 8 && dyl < 8) { collided = 1; }

// After: 1 line
if (sprite_overlap(actor_x[0], actor_yy_lo[0], actor_x[ci], actor_yy_lo[ci], 8))
{ collided = 1; }

Before/After (pong)

// Before: 16 lines of nested ifs per paddle
if (ball_x < 24) {
    if (ball_right == 0) {
        if (ball_y >= p1_y) {
            byte p1_end = (byte)(p1_y + 24);
            if (ball_y < p1_end) { ... }
        }
    }
}

// After: 1 line per paddle
if (ball_right == 0 && rect_overlap(ball_x, ball_y, 8, 8, 16, p1_y, 8, 24))
{ ... }

Closes #421

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