Add stdlib prelude (Power, Ground, NotConnected, Layout, Board)

[akhilles]

Make Power, Ground, NotConnected, Layout, and Board implicitly available in user .zen files without explicit load() statements. Remove now-redundant imports across all .zen files, tests, examples, and docs.

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Note

Medium Risk
Touches core evaluation and symbol indexing behavior by injecting globals into every user module, which can affect name resolution, load order, and tests (notably net auto-naming); broad surface-area changes but scoped to a small, explicit symbol set with opt-out.

Overview
Adds an evaluator stdlib prelude that auto-injects Power, Ground, NotConnected, Layout, and Board into user .zen module scope (with an opt-out flag and automatic disabling for stdlib modules to avoid cycles), and ensures LSP symbol indexing also resolves these implicit symbols.

Refactors test harnesses to materialize embedded stdlib .zen sources in-memory (stdlib_files_for_tests/eval_zen) and updates snapshots to account for changed net auto-naming (including regex sanitization in one CLI test).

Cleans up repo usage by removing redundant load() statements across examples/tests/templates/docs, documenting the prelude in docs/pages/spec.mdx, and dropping the embedded stdlib/docs/spec.md.

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[github-actions]

Build Performance

Board	Base (median)	Head (median)	Change
demo/DM0001	114ms ±6	115ms ±5	+0.7%
demo/DM0002	100ms ±2	101ms ±2	+1.7%
demo/DM0003	109ms ±3	119ms ±2	1.08× ±0.04 slower
arduino/Nano	79ms ±1	85ms ±2	1.08× ±0.03 slower
arduino/UNOQ	159ms ±11	171ms ±3	+7.5%

_{Measured with hyperfine. Times show median ±stddev.}

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Directory name typo "manufactuers" in new stdlib path

Medium Severity

The new file is placed under stdlib/bom/manufactuers/ which is a misspelling of "manufacturers". Since this becomes part of the stdlib import path (e.g., load("@stdlib/bom/manufactuers/wurth_electronik.zen", ...)), fixing this typo later would be a breaking change for any downstream consumers.