feat(types): add core data structures

PackageIndex, Recipe, DepConstraint, BuildPlan, BuildResult, CacheManifest, BinaryCheckResult structs. Pool/BuildSystem/DepOp/Source enums. DepConstraint.parse() ported from ZETA vercmp.lua. TypesException for parsing errors. All @safe, all strings default to empty string.

.omo/notepads/tofu-core/decisions.md

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+# Decisions — tofu-core
+
+Architectural choices and rationales discovered during work on this plan.
+
+_Auto-scaffolded by /start-work. Append new entries below - never overwrite._
+
+---
+
+### Task 4: `version` → `ver` rename
+`version` is a reserved keyword in D (conditional compilation). All struct fields bearing this name are renamed to `ver` — PackageIndex.ver, Recipe.ver, DepConstraint.ver, CacheManifest.ver, BinaryCheckResult.ver. This keeps the API readable while avoiding the keyword conflict. Downstream modules referencing these structs must use `.ver` for the version field.
+
+### Task 4: DepConstraint uses typed `DepOp` enum, not string op
+The Lua reference stores `op` as a string (`nil`, `">="`, `"=="`, etc.). In D we use a typed `DepOp` enum (`ge, le, eq, ne, gt, lt, none`) for type safety and exhaustive switching. The `parse` factory handles string-to-enum conversion at parse time.
+
+### Task 4: BuildPlan is a plain container — no sorting
+The plan specifies that `order()` returns entries in deps-first order *guaranteed by the caller*. The struct itself is just a container with `add()` and `order()` — no topological sort or dependency resolution. Sorting logic belongs in a later module (`tofu.resolver` or similar).
+
+### Task 4: Single exception type `TypesException`
+One exception class for the entire types module — no separate subclasses per error category. The parse failures (bad dep spec, invalid pool) all throw `TypesException` with a descriptive message. Callers catch `TypesException` for all type-parsing errors.
+
+### Task 4: Manual char scanning for dep spec parsing
+Rather than pulling in `std.regex`, the parser uses hand-written `isNameChar()` and `isWhite()` helpers with a simple position cursor. This keeps the module dependency-light (only `std.ascii`). The logic is a direct 1:1 port of the Lua reference patterns: `[A-Za-z0-9_.+-]` for names/versions, whitespace `[ \t]` for separators.
+
+---