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refactor(immut): make RRB-tree radix constants const

Convert the package-private radix hyper-parameters of the immut/array
and immut/vector RRB trees from top-level `let` bindings to `const`:
num_bits -> NUM_BITS, branching_factor -> BRANCHING_FACTOR, bitmask ->
BITMASK (BRANCHING_FACTOR is a power of two, 1 << NUM_BITS = 32).

As consts they are inlined as literals, so the backends see a compile-
time constant divisor for the `len / BRANCHING_FACTOR` and
`len % BRANCHING_FACTOR` operations in the construction/rebalance paths
(operands are non-negative lengths/capacities) and can strength-reduce
them instead of loading a runtime global. The per-element get/set path
already used `& BITMASK` and shifts, so it is unchanged.

Pure rename + let->const; the constants are package-private (no public
API or .mbti change) and no values change. Naming follows the existing
UPPER_CASE const convention used across core (strconv, encoding, ...).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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