Branch: slice/S.8a. Commit only there; never to trunk.

Read first:

• docs/phases/S-small-targets.md, slice S.8: what the tree is for, and the RAM estimate this arena has to meet.
• src/glint/shell/matcher.cr: the style for code that runs both in the image and on Linux: integers, pointers and StaticArray locals, no Array, String building, classes or new of a reference type.
• docs/phases/P-parser.md, P.4b: a StaticArray constant is filled by a start-up initialiser that firmware never runs. The arena must be a value the caller owns (a local or a field), never a constant.

Claim. S.8's program tree lives in a fixed arena of small nodes that needs no heap, so its RAM cost is a number known at compile time.

Passage. S.8's own finding: "Glint's heap (3.4) takes whole 4 KiB pages, a quarter of the Zepto's RAM each. The tree needs a fixed arena that is emptied after each line or after emit, with a stated node limit."

Make: src/glint/shell/tree.cr, module Glint::Shell::Tree.

• Node, a struct of exactly 12 bytes: kind : UInt8, one spare byte, three child indices a, b, c : UInt16, and value : Int32. NONE = 0xFFFF_u16 means no child.
• The kinds this first set needs, as UInt8 constants with a comment each saying which fields they use: INT (value), LOCAL (value = slot 0–7), ASSIGN (value = slot, a = expression), ADD, SUB, LT (a, b), IF (a = condition, b = then, c = else or NONE), WHILE (a = condition, b = body), SEQ (a, b: evaluate a, then b, the result is b's).
• Arena(N), a struct holding StaticArray(Node, N) and a count. add(kind, a, b, c, value) : UInt16 returns the new index, or NONE when full, writing nothing. It also returns NONE, writing nothing, when a child index is not NONE and not an index already added, so a tree can only point backwards and can't loop. [](index), size, capacity and clear. Nothing else.
• Constructors for each kind (int(value), add(a, b), if_(c, t, e) and so on), each a one-line call to add, so specs and S.8c read as the tree they build.

spec/shell/tree_spec.cr checks:

• sizeof(Glint::Shell::Tree::Node) == 12, and an Arena(256) is at most 3,080 bytes (256 × 12 plus the count, padded): 3 KiB of the 16.
• Building the tree for x = 0; while x < 10; x = x + 3; end; x gives the indices and fields the spec spells out.
• A full arena returns NONE and its size doesn't change; a forward or out-of-range child returns NONE; clear empties it and the next add returns 0.

Constraints. No Array, Hash, String building, Pointer.malloc, classes, exceptions or constants of StaticArray. Nothing in the image requires it yet, so image sizes are unchanged. crystal tool format --check passes. Report the line counts.