Prototype int_newtype and typed vec<K, V>

[devin-ai-integration]

Summary

Prototype of strongly-typed integer newtypes for keeping separate index spaces as separate types, plus a key-typed contiguous vector, plus a retrofit of level_ancestor as a worked example.

src/newtype.hpp:

• int_newtype<Tag, T = int> with:
  • implicit conversion from raw integers (per review discussion): from T itself at runtime, and from compile-time constants of any integral type (range-checked via a consteval constructor). Conversion back out (operator T) is explicit;
  • explicit converting constructor between any two newtypes — the inter-space wall is the invariant;
  • ring semantics closed within a type (0 and 1 are distinguished points): + - * / % & | ^ ~, shifts, ++/--, compound assignments, ==/<=>, iostream, std::hash. Raw ints embed implicitly as operands; other newtypes never do;
  • INT_NEWTYPE(name[, type]) macro to mint a fresh tag.
• IntKey concept (IntNewtype or plain integral) + newtype_raw/newtype_underlying_t helpers, so structures can be templated on a key type with int as the default and zero migration cost.
• vec<K, V>: a thin wrapper over std::vector<V> (contiguous) whose operator[]/at accept only K (or things implicitly convertible to it); size() returns the past-the-end key K; push_back/emplace_back return the inserted key; value iteration via begin()/end() and key iteration via keys().

src/level_ancestor.hpp retrofit:

• Now template <IntKey Node = int> struct level_ancestor, with preorder indices in a private internal newtype space pre_t (nested tag): preorder: vec<pre_t, Node>, idx: vec<Node, pre_t>, heavyPar: vec<pre_t, pair<pre_t, pre_t>> (heavy parent, distance). Depth arguments/results (get_ancestor's k, dist) are pre_t too, but raw ints convert implicitly so untyped call sites are unaffected.
• The compiler now enforces the node-id vs preorder-index distinction the old code tracked by convention (e.g. the old a = idx[a] reuse of one variable across both spaces is no longer possible).
• level_ancestor<> with std::vector<int> is a drop-in for the old interface; new tests cover both int and newtype instantiations.

Tests (src/newtype.test.cpp) also include a CSR-graph example where node and edge_slot are distinct spaces (mixing them fails to compile, checked via static_assert), and a values-as-newtypes example (val_t = int32, prefix sums in sum_t = int64 with explicit widening).

Link to Devin session: https://app.devin.ai/sessions/7fff37ee1500445fb693de007d24a2eb
Requested by: @ecnerwala

[devin-ai-integration]

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